One cook's leftovers are another cook's "prep".
What is left over from Sunday's "pot roast" is the foundation for Wednesday's stew, soup or diced/shreaded roast meat and dumplings, or even a hash.
Speaking of hash - I always bake 1-2 more 'taters than I need just for that! You can also grate or cube one for hash-browned potatoes, dice and brown for a fritatta, turn into potatoes O'Brian ...
Leftover baked/boiled/broiled chicken can be turned into chicken with long grain and wild rice with mushrooms.
I've taken leftover grilled hamburger patties - chopped them up (actually, I just crumbled them up in my hands), added them to sauteed onions, bell peppers, and mushrooms, a splash of tomatoe sauce and/or a tetch of Worcestershire and/or steak/chop sauce and used them for a topping for baked potatoes with a little grated cheddar and a dollop of sour cream.
And, that is just a few ideas.
Leftovers doesn't have to be a nasty word .... or a bland meal.
"It ain't what you don't know that gets you in trouble. It's what you know for sure that just ain't so." - Mark Twain