I used to batch cook a week's worth of meals and then microwave them to save energy.
In the oven: a pack of 4 chicken thighs, cooked in a covered dish until tender. I find one of those, when bulked out with other stuff, is sufficient for 2 adults and 2 children. (One for a stir fry with loads of cabbage and veg and noodles or rice. One to make chicken and veg soup, using some of the stock and loads of potatoes and carrots. One chopped up with a tiny bit of bacon, and loads of veg and mushrooms in white sauce to make a pie, topped with pastry or mash. One chopped up with a tiny bit of bacon, sweetcorn and tomato sauce with pasta.)
At the same time, I'd bake some potatoes, scoop out the middles, and mash with butter, cheese and a bit of onion. Ready to microwave later.
I'd also make some pasties and pies. Leek, cheese and potato. Apple and blackberry. Cheese and baked bean. Cauliflower cheese or bolognese if you have leftovers.
A bowl of sage and onion Stuffing and some roast potatoes (goes with the chicken pie).
Flapjacks can be quite cheap. And you can add any last dregs of dried fruit, seeds, chopped apple, nuts, if you have any. And they are great for lunch boxes.
Crumbles are cheap and cheerful, especially if you can get hold of some free apples.
At the end of the week there was leftover soup, made with the contents of the veg drawer, some stock, and carbs (potato/rice/macaroni/pearl barley.)
Basically, most of my meals were a small amounts of meat bulked out with loads of cheap veg, and with a big pile of cheap carbs.
These days I don't have to be quite so careful about the fuel, but we still eat economically.
Pastry Pizza. made with a roll of puff pastry, a jar of sauce and some grated cheese. Plus any toppings left over (ham, onion, tomato, tuna)
Homemade flatbreads with pizza topping (I use SR flour to make the flatbreads)
Potato soup. (Onion, potato, flour, stock, mixed herbs)
Courgette soup (as potato above but with courgettes too).
Veg soup.
Pea and mint soup (Bag of frozen peas, a potato, stock, spoonful of mint sauce/jelly) Blend until smooth.