Combine the mustard, honey, lemon juice, paprika, cayenne pepper, and crushed red pepper flakes together in a small bowl. Remove 3 tablespoons of the sauce and reserve it for later.
Place the chicken breasts into a shallow pan. Pour the marinade onto the chicken breasts then toss to coat evenly. Cover with saran wrap and place into the refrigerator to marinate for 2-3 hours.
Heat a grill pan coated with cooking spray over medium heat. Place the chicken into the hot grill pan and cook for 6-7 minutes; flip and continue to cook for 6-7 minutes, or until cooked through. Let the chicken rest on a cutting board for a few minutes before serving. Drizzle the sauce over the chicken or serve on the side. Enjoy.