Shred potatoes with a food processor or a box grater. With cheesecloth or a clean tea towel, squeeze as much liquid from gratings aspossible.
In a large bowl, mix with onion, egg, flour, salt, and pepper.
In a large heavy-bottomed skillet, heat a half-inch of oil over moderate heat.
Add heaped tablespoons of mixture, frying four or five at a time,for about three minutes per side (smoosh them down with spatula a bit) or until golden brown. Add oil as needed.
Drain on paper towels, and serve right away, or cook ahead, and reheat on baking pans in a 200°F oven.