In a bowl whisk together lemon juice, pepper and salt to taste and add oil in a stream, whisking until marinade is emulsified. With a rolling pin or smooth side of a meat pounder flatten chicken 1/4-inch thick between sheets of plastic wrap. In a large resealable plastic bag marinate chicken in marinade, chilled, 30 minutes.
Prepare grill. Make gremolata: In a small bowl stir together gremolata ingredients. Grill chicken on an oiled rack set 5 to 6 inches over glowing coals until just cooked through, about 2 minutes on each side.
Serve chicken sprinkled with gremolata and garnished with lemon and thyme.