In a medium mixing bowl, combine the balsamic vinegar, olive oil, brown sugar, and Dijon mustard. Add minced garlic, Italian seasoning, salt, and pepper. Whisk until smooth.

Place chicken into a resealable plastic bag or shallow dish. Pour the marinade over the chicken, ensuring each piece is coated. Seal or cover and refrigerate for at least 15 minutes, up to 24 hours for best results.

Preheat your grill to medium-high heat (375°F) or heat olive oil in a skillet over medium heat.

Remove chicken from the marinade and let excess drip off. Grill or cook in the skillet for 6-8 minutes on each side, until internal temperature reaches 165°F.

Let the chicken rest for 5 minutes before serving. Garnish with chopped parsley or basil if desired.