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 fully combined.
02 - Set aside 2 tablespoons of the prepared 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 evenly. Seal and refrigerate for at least 1 hour, or up to 8 hours for deeper flavor penetration.
04 - Preheat the grill to medium-high heat (approximately 375°F to 400°F). Lightly oil the 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, basting with the reserved marinade during the final minutes of cooking. Cook until the internal temperature reaches 165°F and juices run clear.
06 - Transfer the chicken from the grill to a platter and let it rest for 5 minutes. Garnish with chopped fresh parsley and serve immediately.