To ensure that they’re really dry, I place them inside a towel in smaller amounts and tightly squeeze until no more liquid is released. Mix the drained potatoes and onion with the salt, flour, and egg until just combined.
Preheat oven to 250 degrees and line a sheet pan with paper towels or a rack. Pour enough oil to cover the bottom of a large non-stick skillet and heat over high heat until the oil is hot but not smoking.
Using a small scoop or tablespoon, scoop the potato mixture and drop into the pan. Cook, pressing down lightly to flatten the latkes, until the first side is golden brown, then flip them carefully to the other side.