Just thinking it might be helpful to exchange ideas for low cost meals that use little energy.
For me, microwaved jacket potato with beans. If you can afford a little cheese on top and a sliced tomato on side for extra nutrition.
One pot curry with ingredients like lentils or chickpeas, can value tomatoes and some frozen spinach with spices added. Rice. Making extra and have for lunch on a cold winters day.
Homemade tomato sauce - onions, garlic, pepper and value tomatoes. Add whatever you have - kidney beans, chickpeas, cheese, chicken, tuna, mince or quorn mince and add herbs or chilli. Serve with pasta. Again great for lunch or freezing.
If it's just a couple of you, fry eggs, tomatoes, mushrooms, possibly a sausage and serve with beans and slice bread.
Homemade soup made with seasonal veg/veg on offer, onion and stock with bread. Pulses or leftover meat can be added for protein.
If it's a quick lunch or tea, a sandwich with perhaps some tomato/cucumber on the side.