In a medium bowl, whisk together the whole grain mustard, Dijon mustard, honey, olive oil, lemon juice, garlic, paprika, thyme, salt, and pepper. Reserve a 1/4 cup of the marinade for brushing on the chicken.
Marinating the Chicken:
Pour the marinade into a gallon Ziploc bag. Add the chicken breasts to the bag and massage the marinade into the meat until the chicken is well coated. Lay the bag flat on a large plate or baking sheet and place in the refrigerator. Marinate for at least 30 minutes, or up to 8 hours.
Grilling the Chicken:
Preheat the grill to medium-high heat and oil the grates. Remove the chicken from the bag and place on a clean plate. Discard the bag with the marinade.
Place the chicken on the grill and cook, covered, for 5 to 7 minutes, depending on the thickness of chicken. Flip and brush with reserved ¼ cup of marinade, not the marinade you discarded in the bag. Cook for an additional 5 to 7 minutes. The internal temperature should be 165°F.
Allow the chicken to rest for 5 to 10 minutes before slicing or serving.
Notes
To Bake the Chicken:
After marinating the chicken, preheat your oven to 375°F. Spray a large baking dish with nonstick cooking spray. Place the chicken breasts in the prepared pan. Discard the marinade that's in the bag.
Brush the tops of the chicken with reserved marinade.
Bake for 20 to 30 minutes (depending on the thickness of your chicken) or until chicken is no longer pink and internal temperature reads 165 degrees F.