You can store leftovers in the refrigerator for up to 3-4 days. Once the casserole has cooled to room temperature, cover it with foil or transfer it to an airtight container for storage in the refrigerator. When you're ready to reheat the leftovers, you can either warm them up in the oven or microwave until heated through. If the casserole seems a little dry, you can add a little bit of milk or cream to it to help moisten it up.