Make your casserole, but don't bake it or add the topping mixture just yet. Place a piece of plastic wrap directly onto the surface of the potatoes in the casserole dish. Then, for extra protection, add aluminum foil on top and freeze. Before baking, thaw casserole overnight in the refrigerator. Set it on the countertop to come to room temp. while you prepare the filling. Sprinkle potatoes with freshly made filling and bake!