Choose the oven method.

If you have a little casserole left (around two servings), you can warm it in the oven after it has been preheated at 400 to 425 degrees Fahrenheit. Cover it with tinfoil to keep it moist and leave it in the oven for around 10 to 15 minutes.

If you have a larger amount of casserole left, you can put it in the oven for about 20 minutes, at 350 degrees Fahrenheit. Remember to keep it covered with foil so that the casserole can retain its moisture and doesn’t dry out. You can use these temperatures and times for both meat-based and vegetable casseroles. 

Use a meat thermometer to check whether the temperature towards the casserole's center is around 165 degrees Fahrenheit. Heat for five-minute increments.