I'll second the use everything. I make 'stem soup' using the stem of broccoli or cauliflower or both. They taste exactly the same as the florets.
Pay more for 'mature' cheese and use less, it has a stronger flavour so you don't notice.
Protein tends to keep you full for longer so try to have some with every meal.
Buy things like 'cooking bacon' to add to soups.
Do you have a community kitchen / larder near you? The one near me is fab, it costs £5 to join but they give you £5 worth of food.
They get their deliveries on a Monday so that is the best day to get a variety and on a Friday they try to get rid of the stock so will do a 'deal' of a chicken / or a joint of meat with potatoes and veg.
Olio / freecycle / too good to go can be useful.
Plan to have left overs either for the following day or to freeze.
I find having a starter makes me feel more full. I don't mean anything huge, bruschetta made with half a tomato, a small salad, a slice of melon.
I also use packets and tins.
Example of using a tin and planning leftovers.
I have two slow cookers, in the small one I put chicken (equivalent of two breasts but usually thighs). Some sliced mushrooms, a sliced leak, a tin of condensed mushroom soup and a pot of creme fraiche. Sometimes I leave the leek out and use tinned corn. The soup has enough seasoning that you don't need to add any.
I will eat a portion with potatoes or rice or green veg.
What's left over goes into a foil container and I add a pastry lid. This normally goes into the freezer to be eaten at a later date.
If I cook a chicken I eat a portion (or two) I either portion up a couple of 'dinners' with whatever veg I've used or I take the meat off for sandwiches or to freeze. Or maybe to make a curry.
I thin put the carcass back in the slow cooker and add water to make stock.
It's not quite the legendary MN chicken but does a few meals for me.