Preheat oven to 100 degrees C. Grate potatoes in long strips, using smooth strokes to run potatoes across grater into a large bowl of ice water. Using a strainer or slotted spoon, transfer potatoes, reserving liquid, to another large bowl lined with a clean kitchen towel; gently squeeze dry.
Set reserved liquid aside for 10 minutes to allow starch to sink to the bottom; carefully pour liquid from the bowl, reserving milky residue (potato starch) and discard. Transfer potatoes back to bowl with potato starch.
Add onions to bowl; stir in eggs, flour, salt, and pepper.
Line a baking sheet with paper towels; set aside. In a heavy nonstick skillet, heat around 60 mm of oil. Spoon 2 tbsp of potato mixture per pancake into skillet. Make a few at a time, being careful they don't run into each other.
Fry on both sides until golden brown, 4 to 6 minutes. Transfer to prepared baking sheet to drain. Keep warm in oven while preparing the others. Serve hot with applesauce, or sour cream if desired.