Generally, you can use a double layer foil to cover a casserole dish and you can also use a tightly fitted pan Lid. Foil is good because it keeps the surface from cooking faster than the interior of the food.Without proper cover or foil, the food will be under-cooked. The cover lid used to cover the casserole in the oven helps to trap the heat that rises from off food instead of letting it dissipate into the oven.Covering also helps with moistening the food inside like steaming.