Using a 4 sided box grater, or the medium shredding disc in the food processor, grate the potatoes alternately with the onions.
Place the shredded potato/onion mixture into a strainer above a large bowl, and with a wooden spoon, press out the excess moisture. Pour off the liquid from the bowl, reserving the potato starch that remained at the bottom. Place the shredded potato/onion mixture into the bowl and mix to incorporate the starch.
Stir the beaten eggs into the mixture and add the flour or matzo meal to make a light batter.
Add salt and pepper.
Heat ½” oil in a large, heavy skillet over medium-high heat. Drop about 2 tablespoons of batter into the oil and flatten a little. (The flatter the pancake, the crispier it will be.)
Fry until golden brown, about 2 minutes per side, turning only once.