Neither my wife's parents, nor my parents were "rich" monetarely, at least, and so exigencies such as "eating leftovers" were necessary...
Struck by this same "Brand of Cain", we are about the same, and manage to avoid throwing away very little in terms of "leftovers"...
Veggies are the easiest to "predict" in volumes, and I admit no big issue over "dumping" a few spoonfulls of carrots, peas, beans whatever, that I just lack fridge space to bother with...
For mashed potato, you can always boil up a couple "fresh" and add the "cold mashed" in with them, it works pretty good...
Or usethe "cold mashed" to "cut" the sharp taste of mashed turnip/rutabaga, or perhaps use in the "twice baked" sweet potato thing?
With larger, salvageable amounts of veggies, there's always the "steamer basket", or, in a pinch, the microwave...
We tend to make a "ton" of gravy with any given roast or chicken/turkey...there are a few ways to make this "larger in volume" than otherwise, so just using a bit of water, then warming up gravy, then adding leftover meat slices...well there you go with the mashed potato, or a "hot whatever" open faced sandwich...
Sandwiches (cold), obviously...
"Pot Pies" of many flavours and sources...
Pre-prepped and sealed meals for re-heating at work, or "frozen dinners", c/w veggies (requires some "creative" cookware, but the money you save makes this attractive)
Ham in sandwiches, casseroles, pea/bean soups, omelets, "fried ham sandwiches", "chopped ham salads", even homemade pizza, in a pinch!
Chicken/turkey salads, chopped ham salads...
Stir fried "beef and broccholi" (all the "Chinese Food" knockoffs...)(Use that "wok" to stir fry!)
Stews! Chili's!
Grated beef, pork and veal for cabbage rolls...
And, of course, the ultimate last use..."soup's"...or indeed, "blenderised", "stocks"...
All it takes is an active imagination, and a very little effort...plus, I guess, some experience of trying to "stretch" the cooking budget and/or some "hardware" (ie appliances!) to effect the same
And the subsequent meals come out pretty good!
Lifter