Here the great twist is that the mustard and herbs are added after the lamb is grilled, so the flavors stay vibrant. More Amazing Lamb Recipes
Season the lamb racks with pepper. In a large shallow dish, combine the olive oil with the rosemary. Add the lamb and turn to coat. Cover and refrigerate for at least 1 hour or overnight.
Light a grill or preheat the oven to 500°. Season the lamb with salt and grill it, fat side down, until the fat is browned and very crisp, about 5 minutes. Turn the lamb and move it away from the coals (if necessary, push the coals to one side). Cover and grill the lamb for about 10 minutes, turning, until an instant-read thermometer inserted into the meat registers 130° for medium rare. Alternatively, heat a large ovenproof skillet. Add the lamb racks, fat side down, and cook over moderately high heat until well browned, about 8 minutes. Turn the racks and lightly brown the other meaty side, about 3 minutes. Transfer to the oven and roast for about 10 minutes, or until medium rare. Transfer the racks to a cutting board and let stand for 5 minutes.
Brush the lamb racks generously with the mustard and pat the herbs on top. Carve the lamb into 8 double chops. Serve at once.