Make the herb rub and roll the lamb
Combine the parsley or basil, rosemary, lemon zest, garlic, peppercorns, mustard seeds, olive oil and salt in a small food processor (or mortar and pestle) and process to a smooth paste.
Lay the lamb flat on your work surface (it should be roughly rectangular) and trim any visible sinew, but don't trim anything that's holding together sections of the leg. Spread about a third of the rub on the surface of the lamb meat. Roll up the lamb tightly, from one short end to the other, and tie the roast snugly at 1-inch intervals with kitchen twine. Spread the remaining herb rub all over the rolled, tied roast.
Grill-roast the lamb:
Prepare a fire on one side of the grill using enough charcoal to create a fire bed that’s about 6 inches deep and two to three times the width of the roast. Make sure that the vents are clear of ash before lighting the fire. With the grill uncovered, allow the fire to burn down until a light layer of white ash covers some of the coals and the rest are red-hot, 35 to 45 minutes.
Arrange the grate so that the side opening (if there is one) is aligned with the coals in case you need to add charcoal later. Let the grate heat up for 5 minutes and then set the roast on the side that’s farthest from the fire; set an oven thermometer next to it. Cover the grill, close the vents most of the way, and cook.
Check the grill temperature after 20 minutes. It should be between 325° and 375°F. Adjust if necessary by opening or closing the vents (more air will cause the fire to burn hotter and more quickly; less air will make the temperature drop.) If the fire seems to be burning out at any time, add more charcoal and open the vents a bit.
After about 45 minutes, use tongs or a meat fork to turn and rotate the roast to help it cook evenly. Turn the roast so the side that was farthest from the fire is now closest to it, but keep it on the cool side of the grill.
Check the roast with an instant-read thermometer to determine when it’s done; the middle of the thickest section, should read 125°F for medium rare (about 45 minutes after turning the roast). When the lamb has reached the desired doneness, use tongs or a meat fork to roll it briefly over the hot side of the fire to sear the outside, turning it until all sides are seared, about 2 minutes per side. Cover the lamb with foil and let it rest for 10 to 15 minutes before carving.
