Place chicken in a resealable plastic bag, take the air out and use a rolling pin to slightly flatten the chicken.
Be careful not to break the bag.
For the marinade, in a small bowl add Dijon mustard, balsamic vinegar, garlic and thyme and mix until smooth.
Pour over chicken.
Re-seal the bag and refrigerate for 4 to 24 hours, turning the bag occasionally.
Drain chicken and reserve the marinade.
Grill chicken over medium heat for about 7 minutes, occasionally brushing with marinade.
Turn chicken over and brush again with marinade.
Grill for 5-8 minutes or until chicken is no longer pink.
Note: Be sure to fully cook the marinade thats brushed onto the chicken.
If you are concerned about re-using the marinade, reserve some of the marinade before mixing it with the raw chicken and use the reserved marinade for brushing when grilling the chicken.