Potato and dairy can get you suprisingly close. Nutrition is complex and full of unknowns, and like don’t actually do that, but it’s nutritionally not far off on paper.
If I can't vary the recipe (it needs to be the exact same every time), I'd probably live off something inoffensive and bland. Like polenta with fried eggs.
If there's some room for variation however it got to be stew. It's still a stew if I sub potatoes with taro. It's still a stew if I sub the beef with chicken or fish. It's still a stew if I change the seasoning. You get the idea, right?
egg fried rice, I already went about 3 months on just two portions of it per day about a year ago and honestly if you add veg and a bit of meat it's pretty close to giving you everything you need anyway (apart from calories cause I was only getting a little over 1k a day including other snacks).
Would probably go for something very practical. Water would be ideal, as I could get it pretty much anywhere for free or very cheap. Or maybe I'd go for chicken pasta salad, just to have a meal that contains a decent amount of variation.