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 out all the water over a bowl. The potato starch will settle to the bottom. Reserve the starch after you have carefully poured off the water.
Mix the potato and onion with the potato starch. Add the scallions, eggs, matzo meal and salt and pepper.
Heat a nonstick griddle pan and coat with a thin film of vegetable oil.
Take about 2 Tbsp. of the potato mixture in the palm of your hand and shape into a round disc as best you can.
Place the potato mixture on the griddle and flatten with a large nylon spatula. Fry for a few minutes until golden. Flip the latke over and brown the other side. Remove to paper towels to drain.
Serve immediately. (You can also freeze the potato pancakes and crisp them up in a 350°F/180°C oven at a later time.)
Serve with apple sauce or sour cream.