To make marinade, whisk together the balsamic vinegar, garlic, olive oil, mustard, onion powder, salt, pepper, parsley, oregano, and pepper flakes until combined.
Poke chicken all over with a fork. Place the chicken in a dish or zip top bag and pour the marinade over the chicken to cover completely. Refrigerate for up to 4 hours, with a minimum of 30 minutes.
Heat the grill to medium heat and brush with oil to prevent sticking. Place chicken on the grill and cook for 4-6 minutes per side, depending on thickness. Cook to an internal temperature of 165˚F.
Once done, remove chicken from grill and allow to rest for 5 minutes. Then serve warm. Enjoy!
