Pre-heat oven to 425 degrees. Toss potatoes, oil, and rosemary on a rimmed baking sheet. Spread out potatoes in a single layer; season with salt and pepper. Roast, stirring once halfway through cooking, until potatoes are golden brown and crisp outside and tender inside, about 30 minutes.
If you know you are going to be pushed on Christmas Day, then it is possible to half-roast the potatoes the day before (see Preparing Ahead, below). On Christmas Day the potatoes just need to be re-roasted in a hot oven for about 20 minutes.