Prepare marinade in a small bowl by whisking together the mustard, vinegar, salt and pepper. Slowly drizzle in the olive oil, whisking constantly until slightly thickened. Stir in the lemon zest, garlic and rosemary. Store covered in the refrigerator for up to 2 days.
Place lamb in a large bowl or shallow baking dish. Add the marinade and coat the meat well. Refrigerate overnight, covered, turning the lamb once or twice in the marinade.
Remove meat from the marinade 30 minutes before grilling.
Oil the grill and prepare the coals.
Drain the marinade from the lamb and reserve. Grill the meat 4" above the coals, about 20 minutes per side for medium-rare meat, basting frequently with reserved marinade. (Check the lamb for doneness after 30 minutes of grilling).
To serve, cut the lamb into thin slices and arrange on a decorative platter garnished with the lemon quarters and the sprigs of rosemary.