Leftover pie...
Leftover meat (whatever there is, I tend to freeze it so I can mix the meats)
Eggs
Pastry
onions or whatever to add

make pastry case, lay meat inside. whisk eggs and pour over top. add onions etc if used, add pastry top. Bake!

Can leave lid of and make it as quiche style.

Potato cake

left over mash
1 egg
pinch nutmeg
finely chopped onion

mix ingredients together and either fry off or bake

Or good old bubble and squeek.

We don't have leftovers, we have tomorrow's dinner.

Wartime recipe books are great for leftover recipes.