In a large mixing bowl, stir together the mashed potatoes, cheese, scallions, egg and flour until combined.
To make equal size pancakes, use an ice cream scoop or ¼ cup measuring cup. Gently flatten the ball into a thick pancake with your hands.
Place bread crumbs in a bowl. Take one potato cake and dredge in bread crumbs; place in frying pan. Cook until well browned on the bottom. Turn cakes over and cook until browned on the other side.
Add more oil to the pan as needed between batches. Make sure to not overcrowd the pan and do not flip the pancakes too soon or they won't develop a crisp crust.
Transfer the pancakes to a paper towel-lined plate to observe excess oil.
Serve the potato pancakes topped with sour cream
