Grate or finely shred potatoes (you should have 2 cups). Combine potatoes, lemon juice (if you like) and baking soda. Drain potatoes in a sieve or colander for 10 minutes, gently squeezing occasionally to press out excess liquid.
In a large bowl, combine egg, onion, matzo meal, salt, and pepper. Stir in potatoes.
Heat oil in a large skillet over medium heat. Spoon 1/4 cup potato mixture into hot oil, flattening cake into 3-inch oval. Repeat with remaining potato mixture. Cook until golden, turning once. Drain on paper towels. Serve with applesauce. Makes 5 side-dish servings.