Grate potatoes on large holes of a box grater into long strips over a large bowl of cold water. Drain potatoes, reserving liquid, and pat dry.

Let reserved liquid stand for 10 minutes. Carefully pour liquid from bowl, discarding liquid but reserving milky residue (potato starch) at bottom. Stir drained potatoes, onion, eggs, flour, and salt into bowl with starch, and season with pepper.

Preheat oven to 200 degrees. Heat 1/4 inch oil in a large, heavy skillet until it registers 350 degrees on a deep-fry thermometer (or until a breadcrumb dropped into oil sizzles and turns brown in 15 seconds). Scoop heaping tablespoons of potato mixture, squeezing out excess liquid, and carefully place in skillet. (You may need to work in batches.) Press pancakes lightly with a spatula to flatten, and fry until golden brown on both sides, about 6 minutes total. Transfer to a paper-towel-lined baking sheet to drain. Keep pancakes warm in oven for up to 30 minutes. Serve each pancake with a dollop of applesauce and creme fraiche.