Like many moms, when I cook — whether it's fish, casseroles, soups, or even whole-grain pancakes (who doesn't love them for dinner?) — I try to make extras. I label the leftovers and freeze them. This also works for raw ingredients. If I'm chopping an onion, for example, I'll cut up an extra one or two, put the pieces in a plastic sandwich bag, seal that bag in a bigger ziplock bag and pop it in the freezer (then, later, I don't even defrost the chopped onion before tossing it in what I'm cooking). As a general rule, if I have just a bit of something left over, like canned tomatoes or pumpkin, I freeze it. (I keep a larger freezer bag full of smaller freezer bags filled with odds and ends. If the collection begins to grow too big, I go through it and use up the stash.)
