Preheat oven to 250°F. Peel the potatoes and onion. Grate them using the fine grater attachment on your food processor, or grate by hand with the large holes on a box grater.
Transfer grated potatoes and onions to a colander. Press and squeeze to extract and discard as much liquid as possible.
Beat the eggs in a large mixing bowl. Add the grated and squeezed potatoes and onions, flour, salt and pepper. Toss gently with your hands to mix well.
Preheat the oven to 250°F.Place a large (12- to 14-inch) sauté pan over high heat for several minutes. When hot, add oil to a depth of¼ inch. Heat oil until shimmering.
Working in batches, measure¼-cup scoops of the potato mixture into the oil (be careful you don’t splash oil when dropping the batter), leaving enough room between them to turn the pancakes. (You can measure the first one so you get an idea of how large they should be, and then use a large spoon for the rest, or use a 2-ounceice cream scoop.) Flatten each pancake with a flat metal spatula so they aren’t too thick and will cook evenly.
Fry the pancakes one side until golden brown, about 3 minutes, turn with a slotted spatula and fry on the other side for another 3 minutes (lower the heat to medium if they are browning too fast -- before they have a chance to cook thoroughly). Transfer pancakes to a paper-towel-lined baking sheet to drain and place in the oven to keep warm (but don’t stack them or they’ll be soggy). Repeat with the remaining batter, adding more oil, if necessary, between batches. (If freezing for later use, under cook them a bit so they don’t get too dark when reheated and don’t bother putting them in the oven. Let the pancakes cool before wrapping in plastic and freezing in zip-top freezer bags. Defrost, then reheat in a 350°F oven for about 15 minutes).
Serve with applesauce and sour cream.