Preheat oven to 200°C (390°F). Remove the leaves from 4 of the rosemary stalks and place in a motor and pestle with the garlic and salt. Pound until lightly crushed. Add the olive oil and pound until combined. Rub some of the oil over the lamb shoulder, top with remaining rosemary sprigs and secure the shank with string. Place on a rack in a baking dish and roast for 30 minutes. Reduce the temperature to 160°C (320°F). Turn the lamb fat-side down and roast for 1 hour, turn the lamb again and roast for a further 1 hour or until the meat shreds easily off the bone. Serve with remaining rosemary oil.