Finely chop garlic and in a small bowl stir together with remaining herb-rub ingredients. Secure loose flaps of meat, by runnning 2 long metal skewers lengthwise and 2 skewers crosswise through the lamb, bunching the meat together.
Put lamb in a large dish and with tip of a sharp small knife held at a 45-degree angle, cut 1/2-inch-deep slits all over lamb, rubbing herb mixture into slits and all over lamb. Marinate lamb at room temperature 1 hour.
Lightly pat lamb dry. On a lightly oiled rack set 5 to 6 inches over glowing coals grill lamb about 10 minutes on each side, or until an instant-read thermometer horizontally inserted into thickest part of meat registers 125 degrees F for medium-rare. (Alternatively, roast lamb in a roasting pan in middle of a 425 degrees F oven about 25 minutes, or until an instant-read thermometer horizontally inserted into meat registers 125 degrees F for medium-rare.) Transfer lamb to a cutting board. Halve and seed lemon. Squeeze juice over lamb and let stand, loosely covered with foil, 15 minutes.
Cut lamb into slices and serve with any juices that have accumulated on cutting board.