Casseroles are a great way to make a delicious meal with leftovers. When reheating a casserole, it is not necessary to preheat the oven as it can cause the dish to crack. Instead, add the cold casserole to a cold oven and cover it with foil to trap heat and keep it moist. You can also add extra moisture by drizzling broth or dairy into the casserole. Bake at a lower temperature (between 300-350 degrees Fahrenheit) for even cooking. Use a kitchen thermometer to ensure the casserole reaches an internal temperature of 165 degrees Fahrenheit.