I eat a whole-food plant-based diet, which makes combining foods easy: the cooked foods are vegetables, beans (sometimes in the form of tofu or tempeh), and grain (intact whole grain, normally from a batch I've cooked and refrigerated). So I can just combine that into a stew, and generally I whisk together a sauce (lately based on tahini or peanut butter) to dress it up. I sometimes us a checklist, but usually that's redundant since I know the profile I want. https://docs.google.com/document/d/1UITZl-Zehpt6MFd3cPKkacF7AjbQcLm7fkOE855RNhQ/edit?usp=sharing
The leftovers then are what's left of the stew, and I just eat servings of that until its gone. The only "leftover" issue I have to watch for are vegetables I bought that lie in the refrigerator, forgotten or hidden. But I keep on top of that pretty well.
I loved when my mom would use up all the odds and ends and make a big salad for dinner.