How to Make It
Spread the lamb on a work surface. With a boning or paring knife, cut between the muscles and separate them using your fingers. Trim away any excess fat and gristle.
In a large, shallow dish, combine the olive oil, garlic and rosemary. Add the lamb and turn to coat. Let marinate at room temperature for 4 hours, turning a few times.
Light a grill. Season the lamb with salt and pepper; do not scrape off the garlic or rosemary. Grill the lamb over a hot fire, turning often, until an instant-read thermometer inserted in each piece registers 125° to 130° for medium-rare. The times will vary according to the size and shape of the lamb leg, anywhere from 8 minutes for a 6-ounce piece to 20 minutes for a 1 1/2-pound piece. Transfer the lamb to a carving board as each one is done, cover loosely with foil and let rest for 15 minutes. Thinly slice the lamb and serve.