Cut Onion lengthwise to fit feed tube of a food processor, then grate with medium shredding disk. Transfer to a large bowl (do not clean processor).
Peel potatoes and keep in a bowl covered with cold water to keep from discoloring. Working with 1 potato at a time, cut lengthwise to fit feed tube, then grate and add to onion. Toss with lemon juice, then with flour, 2 tsp salt, and 1 tsp pepper. Add eggs. Stir to coat. Transfer to a colander set over a bowl (potatoes will release juices).Preheat Oven to 200 F
Heat 1/4 inch oil in a 12-inch heavy skillet over medium heat until it shimmers. Using a 1/4-cup measure, scoop 4 or 5 mounds of potato mixture into skillet. Flatten with a fork to form 3 1/2- to 4-inch disks. Cook until golden brown, 2 1/2 to 3 minutes per side. Transfer to a towel-lined baking sheet and keep warm in oven while making more latkes.
Note: Cooked latkes can be frozen on a baking sheet, then transferred to a sealable bag or container and frozen for up to 2 weeks. Reheat in a 450F oven for about 5 minutes.