Casseroles are not nearly as complicated as people tend to think. And you don't need a recipe, just a knowledge of combining ingredients. You basically need:
About equal amounts of:
starch (you can replace with a second vegetable or something else if topping is starchy)
something to hold it together (cream soup, sour cream, plain yogurt)-you may need as much as twice as much of this
toppings (cheese, crushed crackers, biscuit dough, hashbrowns, mashed potatoes)
This is a great way to use up leftovers, as long as you keep in mind what tastes well together. Shepherd's pie is a layered casserole, but most of the time, you'll mix the first four things together in a greased dish, season, and cover with the topping.
Had turkey and stuffing last night? Dice up leftover turkey, dice some carrots and onions, mix together in a greashed casserole dish, pour diluted cream of chicken soup over it, season with salt, pepper, and garlic salt, and cover with stuffing. Bake for 30 to 40 minutes (longer if a large casserole) at about 350F. You can also sprinkle cheese over the top of the stuffing.
If you top with mashed potatoes, the casserole should be done when they begin to brown. If you use biscuit dough, insert a knife into the center to make sure the dough is cooked through.
You can make a pot pie in much the same way. Use a nice, deep dish, mix the meat and several vegetables, and add a thick broth. Cover with biscuit dough.
You can also make non-meat casseroles that rely on legumes for the protein, but that's trickier. Unless you use pre-cooked beans and peas or fast-cooking split peas, the topping will probably be overcooked by the time the beans are done. I prefer slow-cooked bean dishes to casseroles.