Preheat oven to 160ºC (320ºF). Sprinkle lamb with salt and pepper. Heat the oil in a casserole or ovenproof dish over high heat. Add the lamb and cook for 1–2 minutes on all sides or until well browned. Add stock, wine, sugar, rosemary and mustard. Cover and roast for 45 minutes. Remove the lamb, add the potatoes to the base and top with the lamb, unroasted-side up. Replace the lid and cook for a further 45 minutes. Slice lamb and serve with potatoes and pan juices. Serves 4.