Very little food is thrown away whole. What gets thrown away is halves: the other half of the onion, the squeezed lemon, four spoonfuls left in a tin, the bunch of herbs that was needed for one sprig. Each of these has an obvious answer, and none of the answers is a recipe.
Half an onion
Cut side down on a plate in the fridge, not in a bag — it keeps three or four days and the smell stays out of the milk. Use it raw and thin in anything that needs a sharp edge, or slice the lot and cook it slowly in oil until soft and sweet. That pan of soft onions keeps a week and improves eggs, sandwiches, pasta and rice with no further thought.
Half a lemon
Squeeze it, even if you do not need the juice. Juice keeps days in a jar and freezes in an ice cube tray; a dried-out half lemon does nothing for anyone. Grate the zest before you squeeze, always. Zest is the part with the flavour and the part that gets thrown away, and it freezes perfectly.
The tin with four spoonfuls left
Tomato paste, coconut milk, chipotle in adobo, curry paste: none of these belong in an open tin in the fridge. Freeze them in tablespoon blobs on a plate, then bag the frozen blobs. You then have single portions for months, which is exactly how you would want to buy them if they were sold that way.
The bunch of herbs bought for one sprig
Soft herbs — parsley, coriander, basil, dill — stand in a glass of water in the fridge like flowers, loosely covered, and last a week rather than two days. Anything left after that goes into oil: chop, cover with olive oil in a jar or an ice tray, freeze. Hard herbs like rosemary and thyme simply dry on a plate and keep for months.
Half a tub of yoghurt, half a block of cheese
Yoghurt going sour is a marinade, a dressing base and the liquid in a flatbread or a cake. It does not need to be eaten with a spoon to be used. Hard cheese, once it is too dry to slice, grates. Grated cheese freezes loose in a bag and goes straight into a pan from frozen. The rind goes in the stock bag.
Halves are where the waste actually happens, and every one of them has a five-second answer: squeeze it, freeze it in portions, stand it in water, or cook it slowly while you do something else.