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 breadcrumbs, 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 to 125 F for medium-rare. Let the lamb sit for 20 minutes before slicing and serving warm.
Notes
Adjusting Recipe for a Smaller Leg of LambIf you don’t have a big crowd, you can definitely make this with a smaller leg of lamb; just adjust the rest of the ingredients down proportionately (and don’t make yourself too crazy with the math — the amounts are really guidelines).You’ll want to reduce the cooking time, too. For medium-rare, you want to take the lamb out of the oven when the internal temperature is at 120 degrees F, and it will climb another 5 to 10 degrees as it sits, which will get you to those rosy pink slices.