In a 9" X 12" rectangular glass baking dish, combine the chopped herbs, garlic, lemon zest, olive oil, salt and pepper. Add the butterflied leg of lamb, turning and coating the lamb. Cover the dish with plastic wrap and refridgerate for 8 to 24 hours.
Bring the lamb to room temperature for at least 30 minutes and up to one hour before grilling.
Prepare the grill with a hot side and a cool side. When the grill is ready sear the fat side of the lamb for 6 minutes and then flip the lamb and sear the other side for an additional 6 minutes. Move the lamb to the cool side of the grill and insert a digital grilling thermometer into the lamb. Open the bottom vents and cover the grill. Cook for about 15-20 minutes or until the thermometer registers 125-130° for medium rare.
Transfer for the lamb to a cutting board and tent with foil. Allow to rest for 10-15 minutes before slicing and serving.