Finely grate potatoes with onion into a large bowl. Drain off any excess liquid. Mix in egg, salt, and black pepper. Add enough flour to make mixture thick, about 2 to 4 tablespoons all together. Turn oven to low, about 200 degrees F (95 degrees C). Heat 1/4 inch oil in the bottom of a heavy skillet over medium high heat. Drop two or three 1/4 cup mounds into hot oil, and flatten to make 1/2 inch thick pancakes. Fry, turning once, until golden brown. Transfer to paper towel lined plates to drain, and keep warm in low oven until serving time. Repeat until all potato mixture is used.