Place potatoes into a baking dish. Season with rosemary, salt, pepper and drizzle with olive oil. Roast in preheated oven for 45 minutes.
Add the roast beef to the potatoes. Season with salt and pepper. Place back into the oven and roast for another 45 minutes.
Meanwhile, prepare the sauce. Heat sugar and water in a saucepan over medium heat until caramelised.
When the caramel begins to set, take off the heat. Add the Maderia and star anise (it will spit). Stir and heat until volume is reduced by half. Add the beef stock, heat through, then add creme fraiche. Pour into a gravy boat and keep warm.
Remove potatoes and roast from the oven. Let beef rest for 10 minutes under a tented piece of foil before carving. Serve sliced beef with potatoes and sauce.