Butter one side of each piece of bread and set aside. In a small frying pan, saute onion slice until soft and golden. It is best to heat the meatloaf in the microwave for about 30 seconds before making your sandwich.
Take bread and place one slice, butter side down, in a frying pan. Sprinkle with parmesan cheese, top with thinly sliced meatloaf and then onion (onion is optional if you are cooking for kids, try tomato slices instead). Sprinkle a little more parmesan cheese on top and then place the other slice of bread, butter side up, on top.
Cook on medium heat in frying pan until golden brown and cheese is melted. Flip and cook other side the same way. Slice and enjoy!!