Preheat the oven to 375°. In a pot of salted water, boil the potatoes over moderately high heat until tender, 25 minutes. Drain and let cool slightly. Peel the potatoes and cut them in halves or thirds. Combine the oil and butter and toss with the potatoes on a large rimmed baking sheet. Spread the potatoes in an even layer, season with salt and pepper and roast for about 1 hour, until browned. Serve hot.