Homemade soup. Onion, plus carrot and/or celery if you have any, chopped & fried until soft. Some garlic makes it more flavoursome - powder, jarred, fresh, from a tube, frozen, whatever. Then just add whatever vegetables you have that are cheap, need using, or you think would taste good. For example, diced potatoes, swede, courgette, aubergine, peas, green beans, broccoli, cauliflower, spinach, salad peppers. I usually add a tin of tomatoes if I can, or a good squirt of puree. You could use canned beans for added protein, or lentils. Add some herbs etc - Italian herbs are noce in this.
If you want, you can add some meat mince at the stage of browning the onion - can be quite cheap fatty mince. This will also increase the protein.
Add something for carbs. Pasta, beans/lentils will help with this too, or you could add rice if you're going to finish it all in one sitting. Or, you could make dumplings. Or just serve with bread. If you make sure there's fat, protein, fibre & carbs in soup, it's pretty filling.
Seconding the idea of pasta bake. We usually just undercook some pasta, make a sauce with whatever vegetables are in the fridge that need using up (or use a jar of a tomatoey value pasta sauce), add some cooked, chopped sausages & mix it all up, then bake for a bit. Or fully cook the pasta & just mix it all through.
Similar with a tuna pasta bake. Condensed (or normal) mushroom soup, some sweetcorn, tin of tuna, some pasta, bit of cheese on top, oven/grill. This is one of DH & DD's favourites.
You can do a "pizza" pasta bake if you can buy some ham, mushrooms, any other pizza toppings you might eat.
You can add cheese to the top of any pasta bake. I would usually add breadcrumbs too, if there's the end slice of a loaf available. Better than wasting it. I put it under the cheese or it can burn under the grill.
I'm vegan (I haven't always been) but for years one of my favourite dishes has been rice & lentils. It's literally just some onion, fried in oil, then approximately equal volumes of rice & red lentils added & cooked in stock. You can add chopped carrots & celery with the onion if you have them, dried herbs or spices, tinned tomatoes or puree, coconut cream etc. If you're really skint it's just oil, onion, rice & lentils. It's good with curry as the rice complement to something, you can make it more soupy if you want & eat with bread. I got the original recipe from a vegetarian student cookbook over 20 years ago.