Preheat your oven to 375°F (190°C) and lightly grease a muffin tin with non-stick cooking spray.
In a mixing bowl, combine the mashed potatoes, cheddar cheese, Parmesan cheese, eggs, green onions, salt, and pepper.
Spoon the mixture evenly into the prepared muffin tin, filling each cup about three-quarters full.
Bake for 20-25 minutes or until the tops are golden brown and the potato cups are set.
Allow to cool slightly before removing from the tin. Serve warm.