01 - In a medium 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 as a basting sauce during grilling.
03 - Place the chicken breasts in a resealable plastic bag or shallow dish. Pour the remaining marinade over the chicken, turning to coat each piece evenly. Seal the bag or cover the dish and refrigerate for at least 1 hour, or up to 8 hours for deeper flavor.
04 - Preheat the grill to medium-high heat (approximately 375°F to 400°F). Lightly oil the grill grates to prevent sticking.
05 - Remove the chicken from the marinade, allowing excess to drip off. Discard the used marinade. Place the chicken on the grill and cook for 6 to 7 minutes per side, brushing with the reserved marinade during the last few minutes, until the internal temperature reaches 165°F and juices run clear.
06 - Transfer the chicken to a platter and let it rest for 5 minutes. Garnish with chopped fresh parsley if desired and serve immediately.