01 - In a medium mixing bowl, whisk together the Dijon mustard, whole-grain mustard, honey, olive oil, apple cider vinegar, minced garlic, paprika, salt, and black pepper until smooth and well combined.
02 - Measure out and set aside 2 tablespoons of the marinade to use later for basting the chicken on the grill.
03 - Place the chicken breasts in a resealable plastic bag or a shallow dish. Pour the remaining marinade over the chicken, making sure every piece is thoroughly coated. Seal the bag or cover the dish and refrigerate for at least 1 hour, or up to 8 hours for deeper flavor penetration.
04 - Preheat your outdoor grill or grill pan to medium-high heat. Lightly oil the grates to prevent sticking.
05 - Remove the chicken from the marinade, allowing any excess to drip off. Discard the used marinade. Place the chicken on the hot grill and cook for 6 to 7 minutes per side. During the final few minutes, baste the chicken with the reserved marinade. Cook until the internal temperature reaches 165°F and the juices run clear.
06 - Transfer the chicken off the grill and let it rest for 5 minutes to allow the juices to redistribute. Garnish with chopped fresh parsley if desired and serve immediately.