In a large bowl, combine the mashed potatoes, shredded cheddar cheese, grated Parmesan cheese, chopped green onions, sour cream, and egg. Mix until all ingredients are well incorporated.
Season the mixture with salt and pepper to taste.
Using a spoon or scoop, portion out the mashed potato mixture and roll it into small balls (about the size of a golf ball).
Roll each ball in breadcrumbs until fully coated. This will give the puffs a delicious crispy exterior.
Place the coated potato puffs on a greased baking sheet, leaving some space between each puff.
Lightly spray the tops of the puffs with cooking spray or drizzle with a little oil to help them crisp up during baking.
Bake in the preheated oven for 20-25 minutes or until golden brown and crispy on the outside.
Remove from oven and let cool for a couple of minutes before serving.
Notes
Serve hot with a side of dipping sauce for extra flavor.