Peel potatoes and coarsely grate into a bowl using large teardrop-shaped holes of a box grater or in a food processor fitted with medium shredding disk.
Add lemon juice and toss to coat.
Coarsely grate onion into same bowl.
Transfer to a kitchen towel (not terry cloth), then gather up corners to form a sack and twist tightly to wring out as much liquid as possible.
Wipe bowl clean, then return potatoes and onion to bowl and stir in flour, eggs, and 1 1/4 teaspoons salt until just combined.
Heat oil in a 12-inch nonstick skillet over medium-high heat until hot but not smoking. Fill a 1/4-cup measure halfway with latke mixture and carefully spoon it into skillet, then flatten to 3 inches in diameter with a slotted spatula.
Form 5 more latkes in skillet and fry until undersides are deep golden, 1 1/2 to 3 minutes. (If latkes brown too quickly, reduce heat to medium.) Turn over and fry until deep golden all over, 1 1/2 to 3 minutes more.
Transfer to paper towels to drain briefly, then transfer to a metal rack set in a baking pan and keep warm in oven. Make more latkes.
