Combine and mix together all ingredients except for the chicken and thyme sprigs in a small bowl.
Place the chicken breast in a 1 gallon Ziploc bag and pour in the marinade into the bag and then toss in the thyme sprigs. Seal the bag and make sure the marinade is fully covering the chicken. Place in refrigerator for 6-8 hours.
Fire up the grill and grill the chicken breasts until no longer pink in the middle or to your likely.
Recipe by Kiss by Brooke at https://www.kissbybrooke.com/2016/08/08/dijon-mustard-and-thyme-marinade-grilled-chicken/