Trim the potato to a 1 ¼ x 2 in/3 x 5 cm rectangle, and slice very evenly 1/16 in/3 mm thick, preferably with a mandoline slicer. Fry at 350°F/175°C until tender and just beginning to brown, 4-5 minutes, stirring often. Drain and let cool. Fry a few potatoes a second time at 380°F/195°C—they will puff up at once. Continue frying until crisp and brown, 1-2 minutes.