In a medium mixing bowl, whisk together ½ cup balsamic vinegar, 2 tablespoons olive oil, 2 tablespoons brown sugar, 2 teaspoons Dijon mustard, 1 tablespoon minced garlic, ½ teaspoon dried thyme leaves, 1 teaspoon salt, and 1 teaspoon black pepper.
Place 4 boneless skinless chicken breasts into a large zip-top bag.
Pour the marinade over the chicken inside the bag and seal it tightly.
Refrigerate and allow the chicken to marinate for at least 30 minutes or overnight.
Preheat the grill to 350 degrees F and lightly brush the grill grates with oil to prevent sticking.
Remove the chicken from the marinade and place it directly on the grill.
Grill the chicken for 5–7 minutes per side until the chicken reaches an internal temperature of 165 degrees F.
Remove the chicken from the grill and let it rest for 5 minutes before slicing and serving.
Notes
Refrigerate the leftovers in an airtight container for up to 5 days. We use an instant-read meat thermometer to make sure that the chicken is cooked to a safe temperature before eating.