The perfect french fry—golden brown, surpassingly crispy on the outside, with a light and fluffy interior that tastes intensely of potato—is not easy to cook.

Here’s how most people do it at home: Cut some potatoes into fry shapes—classic ³/₈-inch batons—and toss them into 375-degree oil until they’re golden brown. This is a mediocre fry. The center will be raw.

Here’s how most restaurants do it: Dunk the potatoes in oil twice, o...