1. Rinse the shredded potatoes in a colander under cold running water and drain for 10 minutes. Squeeze the potatoes in a clean kitchen towel to remove as much liquid as possible. Transfer to a bowl.
2. Add the onion, egg, and matzo meal and stir well. Season generously with salt and pepper.
3. Add enough oil to come ¼ in (5mm) up the sides of a large frying pan and heat over medium heat until the oil shimmers. Add heaping tablespoons of the batter to the oil and flatten slightly into pancakes. Cook, turning once, until golden brown on both sides, about 6 minutes. Using a slotted spatula, transfer to paper towels to drain.
4. Serve immediately, with the applesauce and sour cream on the side, if liked.
German Potato Pancakes: Reibeplatzchen or Kartoffel Puffer are made in a similar way. Squeeze the shredded potatoes over a bowl, reserving the drained liquid. Carefully pour away the liquid and add the starchy residue to the potato and onion mixture. Substitute flour or ground oats for the matzo. Serve with herb butters and smoked salmon for an appetizer, or with a generous sprinkling of sugar as a dessert.
Swedish Potato Pancakes: Raggmunk are similar pancakes but are fried in butter. Omit the matzo meal, onion, and oil. Whisk together 1½ cups whole milk and 2/3 cup all-purpose flour. Stir in 1¾ lb (800g) shredded potatoes (do not rinse), with salt and pepper. Melt 4 tbsp butter in a large frying pan and fry the pancakes in batches if necessary, spreading and pressing each one flat, so they are crisp and thin. Serve with lingonberry preserves and fried salt pork.