Cheese, onion and potato pie: mash, raw onion chopped, grated cheese, into dish, top with more grated cheese.
Homemade fishcakes (make a big vat and freeze for future use): mash, smoked fish of some kind, flaked,(smoked mackerel is cheap if you get the value range, vac-packed), few herbs, finely chopped onion and seasoning. Leave to cool before forming into patties.
Cawl(Welsh soup/stew): leftover roasting joint/carcass (or if you're feeling flush a few bits of chicken or cheap, tough bits of meat that need long slow cooking), put into large pot with couple of chopped leeks, lots of peeled spuds, few chopped carrots, few bits of swede or parsnip, quartered onions and cover with lots of water. Cook for a few hours. We always make this on cold days as it lasts forever and is lush with bread and cheese
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Pesto: use defrosted frozen peas, raw garlic, some parmesan, EV olive oil, seasoning, fresh parsley (or whatever herbs you have in the garden) and blitz. Keeps in the fridge for ages and is LOADS cheaper than real pesto. It's also much less pungent so good for non-hardcore food lovers ;)
Another cheap tip is to buy gammon hocks (Morrissons butchers do them) for a few quid each and use to make terrines/soups, even sandwiches!
xxx