Heat the oven to 400 degrees F.
Slice the potatoes and put on a baking sheet. Roughly tear the rosemary and scatter it over the potatoes; season them with salt and pepper and drizzle with olive oil. Toss the potatoes well to mix all the flavors and put them in the oven. Remove them after 30 minutes and toss them again. Continue to roast them until they are browned and cooked through. Serve immediately.