To reheat tuna casserole from the fridge, take it out of the fridge a half hour in advance, cover it with foil, and bake in a 350 degree F oven for about 30 minutes. To reheat from the freezer, defrost the casserole in the fridge for 24 hours, let it rest at room temperature for 30 minutes, then bake until its internal temperature hits 165 degrees F.