Peel and grate the potatoes using the large holes of a box grater. Peel and finely grate the onion.

Place the grated potatoes in a clean kitchen towel and squeeze out excess liquid. Transfer to a large mixing bowl and add the grated onion.

Add the egg, flour, salt, and optional black pepper. Stir until the potatoes are evenly coated and the mixture holds together.

Add enough oil to lightly cover the bottom of a large nonstick skillet. Heat over medium to medium-high heat.

Scoop a portion of the potato mixture into the skillet and flatten it into a thin, round pancake using the back of a spoon.

Cook until the bottom is golden brown and crisp, then carefully flip. Continue cooking until the second side is golden and the potatoes are tender in the center.

Transfer the pancakes to a paper towel-lined plate or wire rack to remove excess oil. Repeat with the remaining mixture, adding oil as needed.

Serve the German potato pancakes warm with sour cream, chopped green onions, or applesauce, if desired.