Sprinkle lamb chops with pepper and salt. In a small bowl, mix mustard, rosemary, thyme and garlic., Grill chops, covered, on an oiled rack over medium heat 6 minutes. Turn; spread herb mixture over chops. Grill 6-8 minutes longer or until meat reaches desired doneness (for medium-rare, a thermometer should read 135°; medium, 140°; medium-well, 145°).