Butter one side of each slice of bread. Place meatloaf, cheese and any other desired leftover between unbuttered sides. Warm up a large skillet over medium heat. Carefully place both sandwiches in skillet and cook for a few minutes, peeking with a spatula to check their progress. Once the bottoms are golden brown, flip. Press down firmly with spatula on the top of the bread to compress sandwiches (you can also do this by using a lightly sprayed lid from a pot). Remove, and either enjoy right away or reheat at work in microwave or oven.