Preheat the oven to 400°. In a large saucepan, cover the potatoes with cold water. Add a large pinch of salt and bring to a boil. Simmer over moderately high heat until tender, about 12 minutes. Drain.
Return the potatoes to the saucepan and shake over moderately high heat until the potatoes are dry, 10 seconds. Spread the potatoes and rosemary sprigs on a rimmed baking sheet. Drizzle with the olive oil, season with salt and black pepper and toss to coat. Roast for 45 minutes, stirring a few times, until the potatoes are sizzling and starting to brown. Tilt the baking sheet and pour off any excess oil. Roast the potatoes for 15 minutes longer, until browned and crisp.
Meanwhile, light a grill or preheat a grill pan. Season the steaks with salt and black pepper and grill over high heat, turning once, until well-browned and medium-rare, 8 minutes. Transfer the steaks to a cutting board and let rest for 5 minutes. Carve the steaks into thick slices and serve with the potatoes.