1. In a large bowl, combine the mashed potatoes, cheddar cheese, Parmesan cheese, green onions, garlic powder, salt, and black pepper. Mix well until all ingredients are evenly distributed.
2. Add the beaten egg to the mixture and stir until fully incorporated. This will help bind everything together.
3. Using your hands, shape the mixture into small balls, roughly the size of a golf ball. Roll each ball in the breadcrumbs, ensuring a nice even coating.
4. Heat about 2 inches of oil in a deep skillet or saucepan over medium heat. Test the oil with a small piece of potato mixture; if it sizzles and turns golden, the oil is ready.
5. Fry the puffs in batches, being careful not to overcrowd the pan. Cook each batch for about 3-4 minutes, turning occasionally, until they are golden brown and crispy.
6. Remove with a slotted spoon and drain on paper towels to remove excess oil.
7. Serve hot, garnished with extra chopped green onions if desired.

These steps reflect the patience and attention to detail I learned from my grandmother, where the joy of cooking is found in the process as much as the result.