In a large bowl, combine cold mashed potatoes, scallions, cheese and egg.
Add 3 tablespoons of flour. If mixture needs additional flour, add up to 3 more, one tablespoonful at a time until potatoes come together.
For potato mixture into pancakes about 1/4 inch thick.
Place remaining 1/2 cup of flour on a plate and dredge both sides of the pancakes in the flour.
Meanwhile, heat vegetable oil over medium high heat.
Carefully add pancakes and fry in batches until gold brown on both sides, about 3-4 minutes on each side.
Add additional oil if needed between batches.
Season with salt and pepper as soon as they are removed from the frying pan.
Serve with sour cream.