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 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 unusual taste in food.
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. The fun thing is that every Lethal Chef is lethal for a different reason:
Marta only ever cooked for her father, who (being a father) 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 is pointed out and makes a sincere effort to get better, becoming a decent cook in the end.)
Richter had never cooked before at all.
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.
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.
Before Flynn was made playable in the PS3 release, Rita held the title of "Worst team chef" because she considers food more complex than "put in the microwave for five minutes" to be a needless extravagance and a waste of time.
Jean Armstrong of Phoenix Wright: Ace Attorney: Trials and Tribulations has a tendency to make even basic-looking dishes taste terrible. He can't even get coffee right.
Prince Chrom from Fire Emblem Awakening, who basically knocks out his rival Vaike with his horrible food. The Avatar in Awakening can flip-flop between this and Supreme Chef, though s/he is mostly remembered as a pretty bad one. (Panne is the only one who likes his/her cooking.) Sully is famous 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. Chrom's little sister Lissa attempts to bake for Kellam... and he passes out as well.
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.
One of the chop sidequests in Archades involves matching up a dangerously experimental chef with a bored philosopher of cuisine.
The direct sequel 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 "Bad Ass" — 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 in Persona 4 have very questionable cooking abilities, ranging from Rise (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.
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. (The Anime of the Game implies that they put thick starch, three different kinds of pepper, kimchi, and soy milk into it.) The 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.
Yukiko's Joke Ending in Persona 4 Arena features her not only taking out YuNarukami with her cooking by force-feeding him, but she pushes it on her friends as well when they show up. Their expressions (and a very ill looking Yu in the background) says it all.
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. 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.
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, Ken is actually a brilliant chef; once Neku convinces him to cook something more ordinary, his business booms once again.
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.
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.
In The Sims (the first game), 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, 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 from BlazBlue 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..."
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.
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.
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.