Using a mandolin on a large French-cut setting, cut into 1/4-inch batons. Soak them in a large bowl of cold water for at least 1 hour (up to overnight in the fridge) — this pulls out surface starch so the fries crisp.

Drain and dry the potatoes thoroughly with a towel; any surface water will make the oil spatter.

Heat oil to 325°F in a heavy pot or Dutch oven. Fry the potatoes in batches for 5-6 minutes, until pale and soft but not browned — this is the blanching step that cooks the interior. Remove with a spider or slotted spoon and let drain on a wire rack or paper towels. Let them cool to room temperature (10-15 minutes).

Raise the oil temperature to 375°F. Fry the blanched potatoes again in batches for 2-3 minutes, until deep golden and crisp. Drain on the rack, salt immediately while hot, and serve right away.