A meticulously-researched Narnia fic about what happened to Peter in Spare Oom (or, the mundane world) over the summer of the Dawn Treader, when World War II was underway. It doesn't shirk on the difficulties of the war, but it's not dark by any means; one can tell that Peter is guided by Aslan, and his convictions are strong. Rthstewart reinforces and fills out the vivid, strong personalities for every character, including her original characters, which are so well-written you wonder why no mention of them is made in the books.
This is book-based, and is very strongly Original Flavour, which means that there will be religion (albeit to Aslan) and very, very little shipping. Oxfordshire, 1942, can be extremely Christian, so there's a warning for that, but that's solely because of the POV characters, Digory, and Richard. Later stories in the series are much less religious.
On the upside, you get to see what people in Spare Oom who don't know about Narnia think about Peter Pevensie, a 'young' man who shows the qualities of a High King of Narnia. The reaction is usually very heartwarming for the reader.
Oxfordshire, 1942 is Part One of a series called The Stone Gryphon; in the main series, there are two sequels.
Part Two is called Queen Susan In Tashbaan, which focuses on a capable Susan who has not rejected Narnia. Susan goes to America in order to help persuade them to join the fight against Axis in World War II. Her role there is weaved smoothly in and out of real-world events, using a Narnian code. Part Three is Apostolic Way, which is unfinished.
rthstewart has also written a number of other stories which focus on the Narnia that defines her Pevensie siblings, and all are equally recommended. Comedic, heartwarming, thrilling, made of awesome, and heartbreaking by turns, The Stone Gryphon is a fantastic read. If there's one series you read in the fandom, make it be this one. Et cetera and so on, old chap.