Peel 2 lb of potatoes; cut into long french-fry sticks, about 3/8 x 3/8 inches thick. Rinse; shake off water.
Transfer potatoes to a large deep heavy pot, spreading potatoes so they're no more than 2 layers deep. Pour in oil to cover potatoes by 1 inch. Place pot over medium heat. Cook for 15 minutes (oil will begin bubbling gently).
Continue cooking, occasionally loosening potatoes from the bottom of the pot with a heatproof spatula, until potatoes are very tender, 25–30 minutes more. Increase heat to medium-high and cook until golden and crisp, about 15 minutes longer (oil will bubble more vigorously).
Using a slotted spoon, transfer fries to paper towels to drain. Season with coarse sea salt.