This is my favorite way to prepare lamb. It works for roasting in the oven and grilling outdoors. This recipe shows a leg of lamb with its bone, which is a great way to present the meat - in a caveman-esque sort of way. Alternatively, have your butcher bone and butterfly the meat for quicker and equally delicious results. Ideally, the lamb should marinate overnight, so start one day before serving. —TasteFood
Make small incisions all over in the lamb. Insert a few rosemary leaves and a garlic sliver in each incision.
Combine minced garlic, olive oil, mustard, sea salt and black pepper in a bowl and whisk together. Smear all over lamb, distributing evenly with hands. Cover lamb and refrigerate at least 3 hours or overnight. Remove from refrigerator 1 hour before grilling.
If using a barbeque, grill lamb over a low, even fire, turning occasionally until meat thermometer inserted in thickest part of meat registers 130 F., for medium-rare. If using an oven, place lamb in a roasting pan. Roast in a pre-heated 425 F. oven for 20 minutes. Reduce oven temperature to 350 F. Continue roasting until internal temperature of the meat reaches 130 F., 45 minutes to one hour.
Transfer meat to cutting board and tent loosely with foil. Let rest 10-15 minutes before carving. Cut lamb in slices and garnish with fresh rosemary sprigs.
If you choose to de-bone the lamb, ask your butcher to butterfly the meat. This method is ideal for the barbeque. Grill, turning the lamb, until the internal temperature in the thickest part of the meat reaches 130 F., about 25 minutes.