1. If your chicken breasts are thick, place them between two pieces of plastic wrap and pound them gently with a meat mallet or rolling pin until they are about 3/4-inch thick. This ensures even cooking.

2. In a medium bowl, combine the Dijon mustard, honey, olive oil, apple cider vinegar, garlic powder, onion powder, black pepper, and salt. Whisk everything together until the marinade is smooth and well combined.

3. Add the prepared chicken breasts to the bowl with the marinade, ensuring each piece is thoroughly coated. You can also place the chicken and marinade in a large zip-top bag. Seal the bag, pressing out any excess air, and massage the marinade into the chicken. Place the chicken in the refrigerator to marinate for at least 15 minutes, or up to 1 hour.

4. While the chicken is marinating, preheat your outdoor grill to medium-high heat (around 400-450 F). Lightly oil the grill grates to prevent sticking.

5. Remove the chicken from the marinade, allowing any excess to drip off. Place the chicken breasts directly on the preheated grill grates. Close the lid and cook for 5-6 minutes per side.

6. The chicken is done when it has nice grill marks, is opaque throughout, and reaches an internal temperature of 165 F when measured with an instant-read thermometer at the thickest part. Avoid overcooking, as this can make the chicken dry.

7. Once cooked, transfer the grilled chicken breasts to a clean cutting board. Tent them loosely with foil and let them rest for 5 minutes. This allows the juices to redistribute, resulting in more tender and flavorful chicken. Garnish with fresh parsley, if desired, and serve immediately.