Using the large holes of a box grater or food processor, grate the potatoes and onion together into a large bowl. Work quickly so the potatoes don't brown.

Transfer the grated mixture to a clean kitchen towel or cheesecloth. Gather the corners and twist tightly over the sink, squeezing out as much liquid as possible. This step is crucial for crispy pancakes - don't skip it.

Return the squeezed potato mixture to the bowl. Add the beaten eggs, flour, salt, and pepper. Stir until just combined. The mixture should hold together when pressed but not be wet.

Heat 1/4 inch of oil in a large skillet over medium-high heat until shimmering, about 350 degrees F. Drop heaping tablespoons of the potato mixture into the pan, flattening slightly with the back of the spoon. Don't crowd the pan - work in batches.

Fry for 3-4 minutes per side until deep golden brown and crisp at the edges. Adjust heat as needed - if they're browning too fast, reduce to medium. Transfer to a paper towel-lined plate and keep warm in a 200 degree F oven while you finish the batch.

Serve hot with warm applesauce and cold sour cream on the side. These are best eaten right away while the edges are still crackling.