Combine all marinade ingredients. Marinate the lamb, refrigerated for at least 4 - 6 hours, turning at least once. Remove the lamb approximately 1 hour prior to roasting. Discard the marinade.
Preheat the oven to 500°F/260°C.
Place the lamb on a roasting rack in a nonstick roaster and place in the oven. Immediately reduce the heat to 375°F/190°C. Roast lamb for 20 - 25 minutes per pound for medium rare and 30 minutes per pound for well done. When lamb is done, place the meat on a cutting board and let it rest for 10 - 15 minutes. Slice and garnish each plate with additional fresh rosemary to serve.