Do you have an airfyer? I find when cooking for one, using the oven can feel like a faff but an airfryer makes it more doable.
eg chicken thighs - a full tray bake in the oven is too tedious for just me (or DD - who is the person I'm most likely doing this for as an individual) - but I can cook 2 or three in the air fryer in about 25 minutes. I do them just with some lemon, oil, fennel seeds, bit of cumin and cinnamon, dash of whisy/brandy if I have it and some salt but if I was doing it for me/Ds/DH I'd add some vegetables vs cooking veg on the stove separately.
Jacket potatoes are always good with whatever is lying around.
Stir fry for 1 is easy and quick and can be super cheap.
I'd also experiment with pasta sauces - it's quite easy to do small amounts or enough for two nights and that way you can have a lot of variety.
Chicken schnitzel? Cut a large chicken breast horizontally so you have two thinner ones. Dip in flour, egg, breadcrumbs (seasoned with salt, pepper and lemon zest) and shallow fry until golden brown and cooked through. Serve with new potatoes and veg or a salad. Pork works well too.
I buy frozen fish fillets and we sometimes poach those from frozen (they don't look so pretty when you do that, but it tastes fine) or I defrost them and fry those.
French toast with fried mushrooms and cheese. Sounds awful. Is delicious!
Or grilled mushrooms and aubergines, bit of goats cheeze, drizzle of honey in a burger.