In a large bowl, stir together the mashed potatoes, cheese, scallions, parsley, egg, onion powder, and cayenne. Add the flour and mix until just combined.

Place the panko in a shallow dish. Use a ¼-cup measuring cup to scoop the potato mixture, then use your hands to form it into thick patties. If the potato mixture is too sticky to handle, chill it in the fridge for 20 to 30 minutes to help it firm up. Coat the patties in the panko, then flatten until ½-inch thick.

Heat a cast-iron skillet over medium heat and coat the bottom with oil. Working in batches if necessary, add the pancakes and press lightly with a spatula so that the bottoms of the pancakes come in full contact with the pan. Cook for 2 to 3 minutes per side, or until both sides are browned and the cheese is melted.

Garnish with scallions and serve with sour cream.