Prepare the Mixture: In a large mixing bowl, combine 2 cups of cooled mashed potatoes, 2 large beaten eggs, 1 cup shredded mozzarella, ½ cup shredded cheddar, ¼ cup all-purpose flour, 2 tablespoons chopped green onions (if using), and salt and pepper to taste. Mix everything until well combined. Ensure that the mixture is smooth.
Shape the Puffs: With clean hands, scoop out about a tablespoon of the potato mixture and roll it into a small ball. Repeat this step until all the mixture is used, arranging the puffs on a platter.
Heat Oil: In a deep skillet, pour in enough oil to create about an inch of depth. Heat the oil over medium heat until it reaches 350°F (175°C). To check if the oil is hot enough, drop a small piece of the mixture into the oil; it should immediately sizzle.
Fry the Puffs: In batches, carefully add the shaped potato balls to the hot oil. Fry for 2-3 minutes on each side or until they turn golden brown and crispy. Avoid overcrowding the skillet to ensure even cooking.
Drain and Garnish: Once cooked, transfer the puffs to a plate lined with paper towels to drain any excess oil. For an added touch, garnish them with freshly chopped parsley.
Serve: Enjoy these delicious puffs warm, paired with your favorite dipping sauces like sour cream, ranch dressing, or marinara sauce.