In a food processor, combine the garlic, orange zest, thyme and rosemary. Puree, then add the olive oil and blend to make a paste. Smear the paste all over the lamb, place it in a container or deep bowl, cover, and refrigerate overnight.
Preheat the oven to 450°F. Meanwhile, let the lamb sit at room temperature for 30 to 45 minutes. Season the lamb with salt and pepper. In a small bowl, mix together the olive oil, mustard and parsley, add the bread crumbs and use a spoon or your hands to thoroughly blend. Press the mixture all over the top and sides of the leg of lamb, and place it in a roasting pan. Some of the panko mixture will fall off the sides; that’s fine, just kind of tuck it in underneath the sides of the lamb.
Roast the lamb for 15 minutes, then turn the oven down to 400°F and roast for another hour to an hour and 15 minutes, or until an instant read thermometer inserted into the middle of the roast reads 120°F to 125°F for medium-rare. Let the lamb sit for 20 minutes before slicing and serving warm.