Peel the potatoes and put in cold water. Using a grater or a food processor coarsely grate the potatoes and onions. Place together in a fine-mesh strainer or tea towel and squeeze all the water over a bowl. The potato starch will settle to the bottom; reserve that after you have carefully poured off the water. Mix the potato and onion with the potato starch. Add the scallions, egg, and salt and pepper. Heat a griddle or non-stick pan and coat with a thin film of vegetable oil. Take about 2 tablespoons of the potato mixture in the palm of your hand and flatten as best you can. Place the potato mixture on the griddle, flatten with a spatula, and fry for a few minutes until golden. Flip the pancake over and brown the other side. Remove to paper towels to drain. Serve immediately. You can also freeze the potato pancakes and crisp them in a 350- degree oven at a later time.