If I'm reheating a single portion, I will just do it in the microwave for about 1-2 minutes until hot all the way through. But if I'm reheating larger amounts, I think the best way to reheat a creamy chicken casserole is to do it in the oven. Just cover it with foil this time to keep moisture in and let the leftover casserole heat in the oven at 350℉ for about 20-25 minutes until hot.