When traditional French fries are fried, water is rapidly driven out of the starch cells at the surface of the potato, leaving behind tiny cavities. It's these cavities that create a delicate, crispy crust. Since oven fries don't heat fast enough for air pockets to form, we instead coat the potatoes in a cornstarch slurry that crisps up like a deep-fried fry would. We arranged the coated planks on a rimmed baked sheet that we coated with both vegetable oil spray and vegetable oil; the former contains a surfactant called lecithin, which prevents the oil from pooling and, in turn, prevents the potatoes from sticking. Using the oil spray also allowed us to use only 3 tablespoons of oil, just enough to evenly coat the fries. Covering the baking sheet with aluminum foil for the first half of cooking ensures that the potatoes are fully tender by the time they are browned.