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 put in a bowl of cold water.
Cut potatoes lengthwise to fit feed tube, then grate and add to onions. Toss with lemon juice, then with flour, 2 teaspoons salt, and 1 teaspoon pepper.
Add eggs and stir to coat.
Transfer to a colander set over a bowl (potatoes will release juices).
Preheat oven to 200°F. 3
Heat 1/4 inch oil to 360°F in a 12-inch heavy skillet over medium heat. 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 pancakes. Cook until golden brown, 2 1/2 to 3 minutes per side.
Transfer to a paper-towel-lined baking sheet and keep warm in oven while making more latkes.
Cooked latkes can be frozen on a baking sheet, then transferred to a sealable bag or container and frozen up to 2 weeks. Reheat in a 450°F oven (about 5 minutes).
