Put each leg of lamb into a large sealable plastic bag, folding meat if necessary. In a bowl whisk together remaining ingredients and divide between bags. Seal bags, pressing out excess air, and stack in a shallow baking pan. Marinate lamb, chilled, turning it once or twice, at least 8 hours and up to 1 day. Prepare grill. Remove lamb from bags, discarding marinade, and grill on an oiled rack set 5 to 6 inches over glowing coals 8 to 12 minutes on each side (medium-rare for thin parts of meat and rare for thicker parts). (Alternatively, lamb may be broiled 3 to 4 inches from heat.) Cool lamb to room temperature. Lamb may be cooked 1 day ahead and chilled, wrapped well. Slice lamb thin and arrange on a platter. Bring lamb to cool room temperature, its surface covered with plastic wrap. Garnish lamb with herb sprigs and serve with chutney.