Combine the garlic, rosemary and olive oil in a bowl. Season the lamb with salt and pepper, then massage with the garlic and rosemary mixture and put the meat in the bowl. Cover and allow to marinate in the refrigerator for at least 30 minutes.
Allow it to come to room temperature, approximately 30 minutes. Meanwhile, build a fire in your grill, leaving 1/3 of the cooking area free of coals.
When the fire is hot, sprinkle pecan shells or wood chips over the fire (we use lump charcoal and don’t do this step) and place the lamb fat side down. Sear each side of the lamb for approximately 4 minutes each, then remove the lamb to the side of the grill without coals and roast, covered, for approximately 30 to 40 minutes. We shoot for 145 – 150 degrees
Tent in foil for 10 to 15 minutes, and serve.