A few simple tricks -- soaking in warm water and vinegar, dusting lightly with cornstarch, and cooking in a very hot pan -- ensure that the potatoes stay crisp.
Bring water to a simmer in a large pot. Remove from heat, and add vinegar and 2 tablespoons salt, stirring until salt dissolves. Add potatoes, and let sit 1 hour.
Preheat oven to 450 degrees. Drain potatoes, and pat dry. Toss potatoes with 2 tablespoons oil, 2 teaspoons salt, and the cornstarch. Divide remaining oil between 2 rimmed baking sheets, and place in oven until oil is almost smoking, about 5 minutes.
Divide potatoes between baking sheets, and roast until golden brown and cooked through, about 35 minutes, rotating sheets and flipping potatoes halfway through. Season with salt, and serve warm or at room temperature.