Spread the lamb on a work surface. 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. Transfer the lamb to a carving board, cover loosely with foil and let rest for 15 minutes. Slice the lamb and serve.
Note: Original recipe has you separating the muscles and ending up with several pieces that you grill. I chose not to do that, I like the look of the roast. You could do it either way, just be mindful that you don't overcook it. F&W's note for you if you choose to do it their way: 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.