Spoon mashed potatoes into a small pyrex custard cup. If the potatoes are refrigerated, microwave the container until the potatoes are hot and malleable, about a minute.
Using the back of the spoon, make a well in the center of the hot potatoes and crack the egg into it.
Sprinkle cheese over the egg and potatoes.
Bake for 10 minutes or until egg is set to your liking.