Place chicken in a large resealable plastic bag in a shallow dish; set aside.

For marinade: In a small bowl, stir together mustard, balsamic vinegar, garlic, and the snipped or dried thyme until smooth. Pour marinade over the chicken. Seal bag; turn to coat chicken. Marinate in the refrigerator for 4 to 24 hours, turning bag occasionally.

Drain the chicken, reserving marinade. Place chicken on the rack of an uncovered grill directly over medium coals. Grill for 7 minutes, brushing occasionally with reserved marinade. Turn chicken; brush again with marinade. Discard any remaining marinade. Grill for 5 to 8 minutes more or until chicken is tender and no longer pink (170 degrees F). If desired, garnish with thyme sprigs. Makes 4 chicken breast halves.

Some foods need to be grilled hot and fast, some low and slow. To gauge how hot the coals in your grill are, hold the palm of your hand a few inches over the center of the grill rack. If you can hold your hand there two seconds or less, the coals are hot; three seconds indicates medium-hot, four medium, and five medium-low coals.