In a large mixing bowl, grate the russet potatoes using a box grater or food processor. Add salt, black pepper, and cornstarch, and mix thoroughly. Fold in chopped green onions.

Stir in grated mozzarella cheese into the potato mixture, ensuring it's evenly distributed.

Heat a non-stick pan over medium heat and add a drizzle of oil. Allow the oil to heat until it shimmers.

Scoop portions of the potato-cheese mixture into the pan, flattening them into round shapes about 1/4 inch thick. Cook undisturbed for 3-4 minutes until golden brown.

Carefully flip each pancake and sprinkle more mozzarella on top. Cook for another 3-4 minutes until both sides are golden and crispy.

Transfer the cooked pancakes to a serving plate and garnish with remaining chopped green onions. Serve with sour cream or dipping sauce.