Leona Garstein makes food that not only tastes awful, but usually knocks people out. Unlike some, she is aware of this fact. In one scene, she cooks for her boyfriend, and as she cooks she pouts about how it's just going to knock him out again. The first time Leona's boyfriend Tasuki tried her cooking, he needed something to wash it down. Unfortunately for him, Kusuha was the only person nearby. It is implied that this is due to her and Tasuki having opposing tastes in food, such that food loved by one will be loathed by the other.
Kusuha also veers into this territory at times because she likes to add ingredients from her dreaded Health Drink in her recipes. Other than that she's a decent cook.
The classic timeline has a Lethal Chef in form of Tytti Noorbuck, one of the heralds of the Elemental Lords. The problem lies within her sense of taste, since she loves sweet foods, she'll make anything she cooks to become overly sweet... so sweet you could get your stomach swollen by eating it. In addition, cooking is her hobby (though in her defense, she's trying to improve)
A recurring theme in the Tales Series is using cooking as a way to heal after battles. Inevitably, one party member will be a danger to themselves and their teammates the moment they get anywhere near cookware. The fun thing is that every Lethal Chef is lethal for a different reason:
Tales of Phantasia: Arche is just plain bad. In a skit in Tales of the World: Radiant Mythology, it knocked out Kratos, Lloyd and Mormo. The protagonist chokes it all in to avoid the taste. Reid was practically immune to the taste of the food, since he finished his plate fine.
Tales of Symphonia: Raine has a tendency towards bizarrely experimental food combinations ("Spicy cake would be a breakthrough!"). Each character also has a "signature" ingredient that they favor using when they cook, though it's usually an optional thing... Raine's is lemons. Raine's younger brother Genis is a Supreme Chef by contrast, since he says he'd never have survived if he had to eat Raine's cooking all the time.
Marta only ever cooked for her father, who said her cooking was wonderful no matter what. (Though in a break from the normal way this trope works, Marta acknowledges her lack of skill after it's pointed out and makes a sincere effort to get better, becoming a decent cook in the end.)
In contrast, Emil makes incredibly delicious food. In his normal state, he tends to put extreme effort into presentation. In Ratatosk Mode he puts zero effort into presentation, but somehow the food is just as tasty.
Finally, Richter is quite bad because he's never cooked anything in his life. His first dish was for Emil, which caused the latter to pass out from the horrible taste.
Flynn is a fantastic cook... except he tends to deviate from the exact recipe and lacks the instincts and sense of taste to compensate, resulting in a Russian Roulette of either extreme tastiness or extreme vileness because no matter what it tastes like, it always looks fantastic. It's worth noting that Flynn's cooking is often beaten out by a dog. A One Note Chef dog who only makes dog food.
Before Flynn was made playable in the PS3 release, Rita held the title of "worst team chef" because she considers food more complex than "open a banana" to be a needless extravagance and a waste of time.
Tales of the Abyss: Both Luke and Natalia had pampered lifestyles, and never had to cook for themselves before. Natalia misunderstands basic cooking instructions to the point that a stew she tried to make catches fire. Luke isn't quite as bad, but is still a close second because he's an incredibly picky eater. Guy could only comment that Luke's cooking was 'avant-grade'.
The Avatar, AKA Robin, can flip-flop between this and Supreme Chef, though s/he is mostly remembered as a pretty bad one. (Panne and Nah are the only ones who straighforwardly likes his/her cooking.) However, if their supports with Nowi and Severa are anything to go by, they do improve eventually.
Sully is infamous for being this (it's even mentioned in her description on her character roster) and unfortunately passed it down to her daughter Kjelle, who manages to knock out the army with her attempt at breakfast.
Severa herself claims not to be good at the kitchen either, as she explains to her mother Cordelia in their supports. When she gives any of her boyfriends a cake that she baked as a gift, she tells him "Don't you dare complain about the taste!" If her Older and Wiser self Selena from Fates is an indication, however, she improved with time.
It is possible that Robin's bad cooking can be taken as a reference to his/her "predecessor" Kris from New Mystery of the Emblem, who, much like Robin (or at least in the Japanese version), has the tendency to make food that literally tastes like steel, as shown in his/her supports with Caeda (where they apparently somehow make steel-flavored candy, and they mention that their adoptive grandfather threatened to disown them when they wished to cook for him). Though again much like Robin, if the item "Kris's handmade cookies" in Awakening is any indication, they do get better eventually.
The player can build a restaurant in their castle, in which a random one of the units will do the honors of preparing stat-boosting food for the army. While most of them are at least passable chefs, a couple are terrible at it to the point where their cooking can even impact the other characters' stats negatively. Among them are Arthur (absurdly unlucky), Setsuna (too air-headed and a Sheltered Aristocrat so she's probably never cooked before), Keaton (a werewolf with extremely strange tastes), Rinkah (doesn't understand cooking beyond "Throw something on a fire"), Reina (a Blood Knight who has probably never bothered to learn. Plus, she's estranged from her noble parents who wanted her to be a traditional bride, cooking included) and Felicia (extremely accident-prone).
It seems that this is a Running Gag for avatar characters, as Fates's Avatar, AKA Corrin, cooks food that literally tastes like steel... yet again. And it seems that they are even worse than the previous two, as they manage to burn tea. (This is actually fairly easy to do, if you're making tea the old-fashioned pot-and-leaves way and not with modern teabags, and and the taste is very noticeable.)
While never actually shown baking, GLaDOS from Portal lists a cake recipe near the end of the game that includes such ingredients as solid waste, dirt, volatile organic compounds, rhubarb "on fire", needle guns, and preservatives intended to "deodorize and preserve putrid tissue."
This also appeared in Jade Empire, where the main character meets a Lethal Chef named Chai Jin. If you can eat three courses without passing out, each of which actually damages one of your precious health bars, he will reward you... or offer you the chance to go even further, with a truly vile dish he won't even try himself. If you do manage to eat all of his cooking, you are rewarded with a substantial boost to your stats.
One of the chop sidequests in Archades involves matching up a dangerously experimental chef with a bored philosopher of cuisine.
The direct sequel Final Fantasy XII: Revenant Wings reveals that Penelo's cooking is to be feared, though she genuinely tries to get better. Vaan seems to be the only person that likes it, though once she improves he admits he did it by gulping the food down without giving himself a chance to taste it. Several sidequests revolve around getting rare ingredients in an attempt to help Penelo make edible dishes. By the time you complete them all she's actually improved to the point that unbiased parties think she's pretty good.
One of the Persona 3 Social Links involves Fuuka "Tank, I need an exit" Yamagishi, who recruits the main character to taste test her food. The level of Courage the protagonist requires for this is literally titled "Badass" — her cooking is foul enough to chase off nearby animals. On the other hand, as you hang out with her, her cooking gradually improves. As Shinjiro points out, Fuuka's failure in cooking lies in her tendency to not prepare in advance and to try to deviate from the recipe without first understanding the basics, and then panic and further compound her mistakes as a result. For instance, when making chocolate truffles, if the recipe asks for one teaspoon of liqueur, she adds a cup on the basis that she wants a bigger portion to share with everyone. Which is a moot point, since she mistakenly used vinegar...
Three of the main girls have very questionable cooking abilities, ranging from Rise (would be fine if her food wasn't way too spicy), Yukiko (who can make an omelet that, through a process that can only be described as alchemical, has no taste at all) and Chie (just plain awful). Ironically, out of the four females in the main cast, only the Bokukko Naoto could be considered a competent chef (and even with her help, it took three tries to come up with a cake that was deemed suitable for consumption).
When Chie and Yukiko combine their powers, the result is dubbed "Mystery Food X" and manage to take out both the main male leads in one spoonful (Persona 4: The Animation implies that they put thick starch, three different kinds of pepper, kimchi, and soy milk into it). Mystery Food X returns in the fighting game and grabbing it will grant you the Poison status... and you get a trophy for it too.
In P4 Golden, poor Nanako tries to make homemade chocolate for her "Big Bro" on Valentines Day. Normally she averts this trope as she's too young to cook, though she can make eggs sunny-side-up just fine, and normally takes care of the household despite her age. However, on this endeavor... she goes to Chie, Yukiko & Rise for pointers, and finishes it before Naoto could help. The resulting concoction is so vile that it puts "Mystery Food X" to shame; it actually appears to inflict the "Fear" status when you eat it, and looks like a miniature version of the Foul clan Black Ooze demon from the main Shin Megami Tensei series.
Also from P4 Golden, Marie shows up to offer her own Valentine's Day chocolate to the main character just minutes before he runs into Nanako. She has never cooked anything before and she could only use whatever she could find inside the Velvet Room limousine, but still created something that a) smells through the gift-wrapped box, b) is still moving, c) will melt if you let it, d) will run away if it melts, d) actually tastes normal and is incredibly addictive, and e) continues to move in your stomach after being eaten.
The main character can be this if you're not paying attention —which can happen easily enough, since most of the dishes you're asked to prepare are Japanese cuisine which might be unfamiliar to a Western audience. If you mess up, then choose to share your cooking with a school friend during lunch, almost all of them are offended and call you out on it... except Yukiko, who thinks it's hilarious and actually rewards you with points towards her S.Link.
The gag is continued in Persona Q. By combining the cooking of Chie, Yukiko, Rise, and Fuuka, Naoto manages to make an arrow that manages to kill an absurdly powerful FOE in one shot. Mitsuru even wants to take it to a lab to analyze it for weapons development.
Prior to Persona 3 and 4, it's implied that Maya Amano from Persona 2 is an awful chef to the point that Ulala does most of the cooking. To supplement this, Maya's fridge is loaded with canned crab.
Euphoria's cooking in Soul Nomad & the World Eaters is so bad that there is a body count associated with it. It is described as the rest of the cast as nothing short of biological warfare — Which turns into a "Funny Aneurysm" Moment once you've played the game through once and learned that Raksha was using her to develop a concentrated form of Scarlet Iago that was near-instantly fatal.
The World Ends with You contains a subversion with the owner of Ramen Don, Ken Doi. At one point you see his restaurant is floundering due to the presence of a new rival, Shadow Ramen. Neku assumes it's because Ken's food isn't as good - and given he offers you a meal with a visible whole fish, one can understand why he thinks that. However, that meal with a visible whole fish tastes delicious; Shadow Ramen's owner, is just running an under-the-table deal with a blog celebrity, and Makoto's ideas aren't any less weird than Ken Doi's are.
In Metal Gear Solid 2, according to Raiden, Rosemary is one of these. He gives a sigh of relief when he convinces her to book a reservation to a restaurant instead of cooking on their special day, visibly cringing in the CODEC screens when she brings the subject up. And even confides to Snake that rations taste better than her food! This is referenced in the fourth game. Her food still tastes terrible.
In Metal Gear Solid 3, the Soviet chefs responsible for creating the Russian rations were absolutely terrible cooks, as the rations were very bad tasting, and doesn't even do moderate stamina recovery. Heck, even the dogs refuse to eat them if thrown to them.
In Metal Gear Solid 4, Sunny loves to cook eggs, but she always ends up burning them, and both Otacon and Snake, the latter a former Green Beret, are always coming up with excuses not to eat them. Thanks to Naomi, she gets better at it.
In Evolution Worlds, Gre Nade can use his food as a special attack. It damages the enemies and chops their stats.
El Fuerte from Street Fighter IV. Interestingly, he's actually a very good cook, but only when he sticks to Mexican dishes. It's when he tries to experiment that things go horribly wrong as is proven when he mixes chanko stew and borscht, then adds chili peppers and chocolate to serve to E. Honda and Zangief after the tournament, with unpleasant results.
Aurica from the first game is the Less-Than-Lethal version, as though her cooking looks strange, but it actually tastes quite good. At least, that's what Lyner said (though BBQ Soda just sounds weird).
Luca Trulyworth from the second game is a more conventional example. Some of her crimes against food include a dish that knocks out two residents of her own Cosmosphere, two food-based Song Magics (one consisting of flinging food at the enemy, the other a food bomb), a horrendous "Trulyworthit Soup" which still has fish bones and seaweed sticking out of it, and her "Chalonde Trois" which looks somewhat like something from the mind of Lovecraft.
Aeon the chef in Castlevania: Order of Ecclesia is initially presented as this (his longest engagement was for three weeks ), but in the end he makes up for a subversion, since he CAN cook good meals (his miniquests unlock two of the best healing items in the game), he's just too much of a ditz to actually care for his customers or the taste of his food. He still acknowledges his faults and promises to become a better chef, though.
The Vice-Principal, during a School Festival, volunteered herself to work in a restaurant booth. The results were not pretty that the students were practically begging for the Principal to replace the Vice-Principal's cooking.
In an optional scene Jess offers Vayne a bite of her lunch, which is covered in a purple sauce that moves. Given Jess's idea of "medicine," saying no was probably a wise move.
There are a few bad chefs in the Suikoden games. The most prominent is Nanami, who even manages to incapacitate Nash with her cooking in one of the side games. (That doesn't stop one schoolgirl from asking Nanami for for cooking tips to impress a Celibate Hero; the advice ends with "take some bracing tincture and go at it like mad.")There is also Shams in Suikoden Tierkreis, who can only stomach the food made by a lethal chef.
Luminous Arc 2, judging from Roland's reactions, Rina was one, when he's subjected to her cooking in her Intermissions. At one point, she use salt instead of sugar for her cookies. She's actually a rather tragic case, it's implied she can't learn because cooking necessitates being near fire, which she is deathly afraid of.
Dragon Ball Xenoverse has the Supreme Kai of Time. When she offers to cook a banquet for the heroes after the events of the GT Saga DLC, Trunks has to warn the Player Character that her cooking is more destructive than any Shadow Dragon, with Tokitoki flying as far away from the building where the Supreme Kai of Time is cooking, which gives off a menacing glow like the ones possessed warriors give off. In the sequel, We actually see the effects of her cooking when she offers her homemade pudding to Beerus who then shares it with Goku. The pudding nearly poisons Goku and Beerus hates it so much that he goes on a rampage, forcing the player character to stop him.
In The Sims 1, any Sim with 0 points in the Cooking skill will set the stove on fire sooner or later. And if you don't have a smoke alarm, there's a pretty good chance of the fire burning down the house (or as close as game mechanics allow, since the walls won't burn) and killing the entire family, since Sims' default reaction to fire is running right up to it and panicking.
At Stinky's Diner in Sam & Max Beyond Time And Space, Stinky takes great pride in his cultivated talent for producing revoltingly unpalatable foods, and his granddaughter shares his gifts. Sam and Max don't actually eat there, they just order the most implausible things they can think of.
Sam: We'd like to waste time by ordering stuff you don't have.
Stinky: Knock yourselves out.
Sam: Crab and salamander enchiladas in green sauce.
Stinky: [yelling to the kitchen door] Sal, I want Lou Dobbs and Greta Van Sustern doing double-dutch on the back of a rogue elephant!
Sam: Chicken chow mein with chocolate covered raisins and a caramel swirl.
Stinky: [shouting] Sal, get me a hyperactive spider monkey in a powder-blue cardigan. And why don't we go ahead and wrestle him to the ground and tickle him until he pees.
Max: I have got to see what goes on in this kitchen!
At one point they actually do order something, a sandwich made with two slabs of Basalt Rock. They're not the ones who eat it though.
Enchanted Arms gives us Karin's "special stew" just the sight of which causes other characters react with shock and horror (it's hidden with a mosaic censor even!) and after Atsuma just tasted it he blacked out and believed he had died. See the 5:30 mark here
Yonah, NieR's Ill Daughter, has tendencies of this and Cordon Bleugh Chef, mixing the most diverse ingredients into noxious meals that only her father would willingly (and lovingly) consume —even if he's thinking to himself how they "taste like death."
NoelVermillion is an awful cook. The poor girl actually loves to cook and pours her heart and soul into every dish she makes, but her sense of taste and smell are so hideously bad that even the resident Eldritch Abomination is knocked unconscious by her food. The only two people capable of eating her food are Hazama (Rule of Funny) and Remix Heart character Mai Natsume, who tastes the "love" that Noel put into her food (which later becomes a recurring gag that she adores eating her food). Bang decided to eat her cooking despite its unpleasant appearance. He exploded.
(Description of Noel's cooking): "What that 'something' was defied description. It didn't sit on the plate so much as squat malevolently, hating all and being hated by all..."
A short story about pre-Terumi Ragna and Jin reveals that Saya is one as well. Her attempt at making raspberry jelly did the same thing to Ragna that Noel's cooking did in the aforementioned gag reel.
Antoine from Dead Rising 2 is a LITERAL Lethal Chef. After going mad since his interviewer came late (actually dead), he decided to cook up a meal for him/her using human meat. Right after explaining this to you, he goes batshit crazy starts throwing knives, plates, frying pans and even attempts to choke you by forcing an apple into your mouth. Later, he dies by slipping and falling face-first into a pot of extremely hot oil.
Deadly Premonition has Emily and her unappetizing but earnest "Nice Try!" cooking, although it is heavily implied that her food is much worse than York tries to put it. In fact, it seems she is completely inept at cooking in general, as indicated by the large scorch mark in her kitchen (which Emily attributes to a towel landing in a frying pan... while she was reaching for the mayonnaise), her food being described as "unfit for anything with less than four legs," and York's joke about how Emily's composter appears to be illegal dumping of industrial waste. There is even an in-game item, "Emily's Bagel Sandwich," which recovers some health and energy at first, but causes both to drop like a rock later. Several side quests involve York helping Emily improve her cooking by bringing her ingredients (but even then does it turn out how far off the mark her cooking is).
Emily: ...enough. But how did you know that cheese was the answer?
York: I was looking at the ingredients, and it hit me. Milk, macaroni, flour, butter... you were trying to make "Macaroni and Cheese", right?
Emily: [Irritated] Macaroni and Cheese...? Of course not! It's "Italian Beef Stew!"
York: ... Zach, have you ever heard of a beef stew with no beef in it? I certainly haven't... at least not until today.
Reyna from Vanguard Bandits is more than capable of making dishes that make people vomit uncontrollably or burn their insides. Only the Big Eater Barlow is immune to the pain. But he also finds her brownies are just as good going down as they are coming back up.
Natsumi in Inazuma Eleven, being an Ojou, had hired help to cook for her, so she had no culinary experience whatsoever. On her first attempt at making food, the result looked normal, but it made Endou turn blue and sweat profusely when he ate it. In spite of the fact that Aki and Haruna were both walking her through it. And the fact that she was trying to make onigiri - the recipe for which is basically "add salt to rice, wad it around whatever filling you want, and wrap it in a piece of nori". Inazuma Eleven GO shows that ten years later, she had learned how to make food that looks delicious... but it still tastes terrible.
Swampling in Simon the Sorcerer and his trademark Swampling Stew. Somehow, he manages to make a franchise out of it by the second game.
Granny's diary: It was my granddaughter who cooked again today. Putting that to the lips shortens the life! I thought of a way to get by without eating. I'll try it tomorrow. I just hope I'm not caught.
Mass Effect 2 zig-zags the trope by presenting us with Normandy chef Mess Sergeant Rupert Garder. According to the crew talk when you first come aboard, Gardner is the worst chef ever. Then you actually talk to the guy, and find out he's actually quite good, it's his ingredients that are bad. A quick pit stop at the Citadel can remedy this situation.
Hawthorne: Hey Rupert, new food is great! Tastes like you added more food, less ass!
In Adventure Bar Story and its sequel, Adventure Bar Labyrinth, Kamerina is a terrible cook, so her sister Siela (the main character of the first game, who can whip up almost anything you ask her to) asks her to man the front desk and stay out of the kitchen.
Vishnal from Rune Factory 4 should not be allowed anywhere near a kitchen. Ever. This is problematic, as he aspires to be a butler and believes a butler should be able to do anything for his master, including cook. Forte, the female knight is also an example, but she acknowledges that her cooking it lethal, and uses the lethal concoctions in battle.
One of the Logar village quests in Runes Of Magic involves fetching beetle guts so that this woman named Gil can make soup. When you deliver said soup to her love interest Del in the following quest he comments that it took him three days to recover from her bear-eye wine.
Chef Nanor from Dungeon Of The Endless. While he is able to cook well to support his teammates, he also has a skill where he cooks something so bad that the gas deals Damage Over Time to all enemies in the dungeon* In comparison, most other DoT skills only hit enemies in the room while also making them have a higher chance of dropping Dust.
Papyrus from Undertale only knows how to cook spaghetti, though that's using a very generous definition of "knows". He learned from Undyne, who struggles to make spaghetti without burning her own house down.
Sans: i bet if he keeps it up, next year he'll even make something edible.
A dropship conversation in Evolve reveals that Hank has cooked grilled canyon eel. This wouldn't be so bad, except for how it was still alive when served.
Linca from Atelier Ayesha: The Alchemist of Dusk spent most of her life training to be a swordsman, but her awesome sword skills come at the expense of almost everything else, especially cooking. In a Side Quest where Ayesha teaches her the basics of cooking, Linca somehow manages to turn a simple egg into a clump of dirt with a flag on it. She misinterprets the lessons she learned from Ayesha and winds up getting Marion (the woman Linca is bodyguarding) sick.
Noel's aforementioned lack of skill carries over to fan game Rakenzarn Tales, where she drags first-time chef Dark Magician Girl into it. The rest of the brigade is terrified of having her cooking and Kanon refers to it as "life-threatening". It's best shown at the start of Chapter 9 where the two try making cream soup. The end result is Kyuu running screaming for the bathroom to throw up while Kite and Kanon faint from it. Apparently, they didn't know when to stop with adding the spices.