Place chicken in a resealable bag or a bowl. In a mixing bowl, whisk together balsamic vinegar, olive oil, garlic, rosemary, thyme, dijon mustard, salt, and pepper. Pour marinade over chicken, seal the bag or cover the bowl, and refrigerate for at least 2 hours or up to 24 hours.
Preheat grill or oven to medium-high heat. Remove chicken from marinade and shake off excess. Grill chicken for about 5-7 minutes per side or until fully cooked and internal temperature reaches 165°F. Alternatively, bake in the oven at 400°F for 20-25 minutes.
Garnish with chopped parsley (if using) and serve warm.
Notes
Best enjoyed fresh but can be stored in the refrigerator for up to 2 days.