Pat the chicken breasts dry with paper towels. If breasts are thicker than 3/4 inch, butterfly or pound to even thickness to ensure consistent marinade penetration and cooking.
In a non-reactive bowl, whisk together balsamic vinegar, olive oil, and honey until well combined (approximately 45-60 seconds of vigorous whisking).
Whisk in Dijon mustard until the mixture visibly thickens. Then add minced garlic, chopped rosemary, thyme leaves, salt, black pepper, and red pepper flakes if using. Continue whisking for another 30 seconds until fully incorporated.
Place the chicken in a resealable plastic bag or shallow glass container. Pour the marinade over the chicken, ensuring all surfaces are coated. Seal the bag, removing as much air as possible, or cover the container.
Refrigerate and marinate for 2-8 hours (4 hours is optimal for most preparations). Turn the chicken or massage the bag halfway through the marination time to ensure even flavor distribution.
Remove chicken from the refrigerator 20-30 minutes before cooking to allow it to come closer to room temperature. This ensures even cooking. Remove chicken from marinade, allowing excess to drip off, but do not rinse.
Cook using your preferred method (grill, bake, pan-sear, or air fryer) until the internal temperature reaches 165°F (74°C) at the thickest part.
Allow the cooked chicken to rest for 5-7 minutes before slicing to allow for juice redistribution.