In a large mixing bowl, combine your cold mashed potatoes, shredded cheddar cheese, finely chopped green onions, garlic powder, salt, and black pepper. Use a spatula or your hands to mix everything together until the mixture is thick and sticky.

With your hands, scoop out heaping tablespoons of the mixture and roll them into balls about the size of a golf ball. Place each formed potato ball on a tray lined with parchment paper. Refrigerate them for at least 20 minutes.

Set up your breading station by placing beaten egg in one bowl and breadcrumbs in another. Dip each chilled potato ball first into the beaten egg, then coat thoroughly with breadcrumbs.

In a deep skillet, heat oil over medium heat until it reaches 350°F (175°C). Carefully add the breaded potato balls in batches, frying for 2-3 minutes or until golden brown and crispy.

Serve warm on a platter with your favorite dipping sauces.