Ingredient substitutions: Use homemade gravy made from a roux or cream of mushroom soup if you don’t have gravy mix on hand. You can add vegetables such as carrots and potatoes for a complete meal. Leftovers can be stored in an airtight container in the refrigerator for up to 4 days. Reheat gently in the microwave or on stovetop, adding a splash of water to loosen the gravy if necessary.