Grate potatoes and onion using the fine side of a box grater or a food processor. Transfer to a large bowl and let sit for 2 minutes to allow liquid to release.
Squeeze the grated mixture in small batches using a clean kitchen towel or fine mesh sieve until most liquid is removed. The remaining moisture should be minimal to ensure crisp pancakes.
Return solids to bowl. Add beaten eggs, flour, salt, and pepper. Mix gently until just combined - the batter should be cohesive but not soupy.
Warm a heavy skillet over medium-high heat and add a thin layer of oil. When oil shimmers and smells neutral, spoon 2-3 tablespoons of batter per pancake into the pan, flattening slightly with a spatula.
Fry until deep golden and crisp on the first side, 3-4 minutes, then flip and cook another 3 minutes. You should hear a steady sizzle and see lace-like edges forming.
Transfer cooked pancakes to a wire rack set over a baking sheet to stay crisp. Keep warm in a 90 C (200 F) oven if making in batches.
Serve hot with sour cream or applesauce, garnish with chives or smoked fish if you like. The aroma will be savory, with a toasty, caramelized onion note and a tender center that yields when you bite.