Preparation Drop garlic down feed tube of mini processor; chop finely. Scrape down bowl sides. Add pancetta, rosemary, and pepper; blend to coarse paste. Using small sharp knife, make 1/2-inch-deep slits all over lamb and fill each with pancetta paste. Spread any remaining paste over outside of lamb. Place lamb, seam side down, in roasting pan. Do ahead Can be prepared 1 day ahead. Cover with plastic wrap and chill. Preheat oven to 300F. Roast lamb uncovered until very tender and thermometer inserted into center registers 170F, about 5 hours. Transfer to platter; let rest 15 minutes. Cut lamb crosswise into 1/2-inch-thick slices. Pour pan juices into bowl. Remove and discard fat from surface. Spoon pan juices over lamb.
Preparation Drop garlic down feed tube of mini processor; chop finely. Scrape down bowl sides. Add pancetta, rosemary, and pepper; blend to coarse paste. Using small sharp knife, make 1/2-inch-deep slits all over lamb and fill each with pancetta paste. Spread any remaining paste over outside of lamb. Place lamb, seam side down, in roasting pan. Do ahead Can be prepared 1 day ahead. Cover with plastic wrap and chill. Preheat oven to 300F. Roast lamb uncovered until very tender and thermometer inserted into center registers 170F, about 5 hours. Transfer to platter; let rest 15 minutes. Cut lamb crosswise into 1/2-inch-thick slices. Pour pan juices into bowl. Remove and discard fat from surface. Spoon pan juices over lamb.