Place the chicken breasts within a folded piece of plastic wrap; sligtly flatten upper portion of each breast with the broad side of a chef's knife to promote even grilling. Place breasts in a shallow glass or ceramic dish. Place mustard, vinegar, garlic, honey, thyme, salt and pepper flakes in a small bowl; stir with fork to combine. Add oil a little at a time and whisk to combine marinade. Pour marinade over breasts. Cover with plastic wrap and marinate in refrigerator,turning once or twice in marinade, for at least 2 hours, or up to 4 hours. Remove breasts from marinade, scraping any bits clinging to chicken back into the shallow dish. Transfer all marinade to small saucepan and bring to a boil; reserve. Lightly grease grill rack with cooking spray. Preheat grill. Place breasts on grill. Cook covered with lid, basting frequently with marinade, until tender, approximately 5 to 6 minutes on each side.