In a small bowl, whisk together the whole grain mustard, honey, lemon juice, minced garlic, paprika, salt, cayenne pepper, and red pepper flakes until smooth.
Reserve 4 tablespoons of the honey mustard sauce for serving.
Pour the remaining marinade over the chicken breasts and coat evenly.
Cover and let the chicken marinate for 30–45 minutes at room temperature.
Preheat the grill to medium-high heat.
Remove the chicken from the marinade and discard the used marinade.
Grill the chicken for 6–7 minutes per side, or until the internal temperature reaches 165°F (74°C).
Transfer the Grilled Honey Mustard Chicken to a serving platter.
Spoon the reserved honey mustard sauce over the chicken.
Cover loosely with foil and let rest for 5 minutes before serving.