Step 1: Prepare the Potato Mixture

1. In a large mixing bowl, combine the mashed potatoes, shredded cheddar cheese, Parmesan cheese, beaten egg, and green onions (if using).
2. Add the garlic powder, onion powder, salt, and pepper. Mix well until all ingredients are evenly incorporated.
3. Gradually add the flour, 1 tablespoon at a time, until the mixture is firm enough to hold its shape when pressed. The amount of flour may vary depending on the consistency of your mashed potatoes.

Step 2: Shape the Pancakes

1. Scoop about 2 tablespoons of the potato mixture into your hands and roll it into a ball. Flatten the ball into a patty about 1/2 inch thick.
2. Repeat with the remaining mixture, placing the formed pancakes on a baking sheet or plate.

Step 3: Heat the Oil

1. Heat the vegetable oil in a large nonstick skillet or cast-iron pan over medium heat. The oil should shimmer but not smoke.
2. Test the oil by adding a small piece of potato mixture—if it sizzles, the oil is ready.

Step 4: Fry the Pancakes

1. Place the potato pancakes in the skillet, leaving space between each one to ensure even cooking. Fry in batches if necessary.
2. Cook for 3-4 minutes on each side, or until golden brown and crispy. Flip gently using a spatula to avoid breaking the pancakes.
3. Once cooked, transfer the pancakes to a plate lined with paper towels to drain excess oil.

Step 5: Serve and Enjoy

1. Serve the cheesy potato pancakes warm, topped with a dollop of sour cream, a sprinkle of parsley, or your favorite toppings.
2. Pair with eggs for breakfast, a side salad for lunch, or a hearty protein for dinner.