Prepare the mixture
In a mixing bowl, combine the leftover mashed potatoes, shredded cheese, flour, egg, green onions (if using), garlic powder, and black pepper. Mix until fully combined.
Shape the pancakes
Scoop about 1/4 cup of the mixture and shape it into a round, flat pancake about 1/2 inch thick. Repeat with the remaining mixture.
Heat the skillet
Heat the vegetable oil or butter in a large skillet over medium heat until hot.
Cook the pancakes
Place the pancakes in the skillet, leaving space between each one. Cook for 3-4 minutes per side, or until golden brown and crispy. Adjust the heat as needed to prevent burning.
Serve
Remove the pancakes from the skillet and place them on a paper towel-lined plate to drain excess oil. Serve warm with sour cream, ketchup, or your favorite dipping sauce.