Peel the russet potatoes and onion, then grate both on the large holes of a box grater or in a food processor fitted with the shredding disk. Drop the shreds straight into a large bowl of cold water as you go so they do not turn gray.

Drain the shreds in a colander, then pile half onto the center of a clean kitchen towel. Gather the towel into a sack and twist hard over the sink until no more liquid drips out and the shreds feel noticeably lighter; repeat with the second half.

Let the squeezed-out liquid sit in a bowl for 2 minutes, then carefully pour off the clear water on top. A chalky layer of potato starch will be left at the bottom: scrape it back into the dry shreds (this step is optional but helps the latkes hold together).

In a large ceramic mixing bowl, combine the potato and onion shreds with 2 large eggs, 1/3 cup flour or matzo meal, 1 1/2 teaspoons kosher salt, and 1/2 teaspoon black pepper. Mix gently with a fork just until the mixture looks like a loose, shaggy batter that clumps when pressed.

Pour 1/2 inch of vegetable oil into a heavy skillet and set it over medium-high heat. Heat until a thermometer reads 350 to 375 degrees F, or until a small drop of the mixture sizzles the instant it touches the oil.

Working in batches of four so the pan is never crowded, scoop 1/4 cup portions of the potato mixture into the hot oil. Flatten each one gently with the back of a spoon into a round about 3 inches wide.

Fry the latkes for 3 to 4 minutes on the first side, until the edges turn deep golden and lacy, then flip with a slotted spatula and fry the second side for another 3 to 4 minutes. They are ready when both sides are a dark, even gold and feel firm, not soft, when pressed.

Lift the latkes straight onto a wire rack set over a rimmed baking sheet to drain; skip paper towels, which trap steam and soften the bottom. Sprinkle with a little extra salt while they are still hot.

Let the oil come back up to 350 degrees F between batches and skim out any loose potato bits with a slotted spoon before adding the next round. Repeat the scooping, frying, and draining with the remaining mixture, adding a little more oil to the skillet if the level drops.

Pile the finished latkes onto a platter and serve warm, with small bowls of applesauce and sour cream alongside for spooning over each one.