You want to base you main meal on at least 3-5 veg.
If you make spag bol, add onion, finely minced celery and carrot, then mixed peppers and maybe mushrooms as well as chopped tomatoes and mince. It takes longer to prep but really bulks out the dish so you often have enough for 6 or portions from a 400-500g pack of mince.
Cut strips of chicken breast or thigh and stir fry them with peppers, beansprouts, spring onions, finely sliced carrots, celery if you like it, and pak choi or chinese leaf. You can add baby corns and mange tout or sugar snaps but these are very expensive for the amount of veg you get so stick to the cheaper bulkier veg if you want to keep cost down.
Risotto with butternut squash, red onions and red peppers, smoky paprika and chickpeas is lovely. You can add chorizo if you want.
Fritatta - fry onions, finely chopped potato, peppers, peas, garlic and herbs you like. When cooked, pour beaten eggs over them into the frying pan and cook until base is firm then finish with some grated cheese on top, under the grill.
Soup is so easy. I just fry onions or leeks, then add carrots and any other left over root veg - potatoes, parsnips, squash etc with some stock. Throw in half a cup of red lentils or a tin of butter beans. Use garlic and either herbs or spices to flavour it (chilli and ginger with cumin or rosemary, sage and thyme) When the veg is very soft blend it with a hand blender. If it's too thick, add more stock.