Remove the lamb from the fridge one hour before you want to cook it, to let it come to room temperature. Preheat the oven to 200ºC. To make the marinate, peel and crush three cloves of garlic and add to a bowl. Roughly chop the rosemary leaves and add to the garlic. Finely grate in the lemon zest and drizzle in a good lug of oil, then mix everything together. Season the lamb with sea salt and black pepper, then drizzle with the marinade and rub all over the meat. Cook the lamb for one hour and 15 minutes if you want it pink, or 1 hour 30 minutes if you prefer it well done. When the lamb is cooked to your liking, remove from the oven and leave to rest for 15 minutes.