Preheat oven to 450 degrees. Cover potatoes with 2 inches of water in a medium saucepan and bring to a boil; add a generous amount of salt. Boil until potatoes are just tender, about 5 minutes. Drain and immediately toss in a bowl with oil and rosemary. (Potatoes will break apart slightly.) Season with salt and pepper.
Transfer potatoes to a rimmed baking sheet. Roast, flipping once, until golden and crisp, about 35 minutes. Serve immediately.