1combine the grapefruit juice, clementines, wine, garlic, rosemary, two tablespoons black ground pepper, and the oil in a blender or processor, and pulse till the mixture forms a paste. Place the leg of lamb in a large, shallow, nonreactive dish or pan. rub the marinde paste over it, and turn the lamb on all sides to coat it. Cover, and refridgerate for at least 12 hours(up to 36 hours). Remove the lamb from the marinade, reserving the marinade, and transfer to a roasting pan. season the lamb aggressively with salt an pepper, and let it stand room temp for 45 min, meanwhile, preheat oven to 425 degrees f.