Casseroles are good food. Rob loves them, so I love them. They are also a wonderful way to use up leftovers (oops. sorry, darling -- EXPERIENCED food
) My rule of thumb for this is...
Think of the meal you serve in its separate elements -- the protein, the veg, the starch. What goes together in that form goes together in a casserole, and all you need to add is a cup or two of a complementary sauce, and something to dress the top.
Layer the elements or mix them together, as you prefer. If you need more veg, saute some onion or mushrooms or peppers, or add a can of corn.
Make your sauce from a package, or a soup, or from scratch, or use leftover gravy. Extend the leftover gravy, if necessary, with a gob of sour cream, or a splash of wine, or the forlorn leftover of hollandaise sauce.
Bread crumbs, cracker crumbs, cheese or whatever on top. Since you are doing this with leftovers, everything is already cooked and just needs a little time together in the oven to get acquainted -- 30 minutes or so at 350, just long enough to get the sauce bubbling, and then a few minutes under the broiler, if the top hasn't browned enough -- and dinner is ready.
There isn't much in my leftovers bin right now or I'd do one tonight!