Combine the mustard, honey, lemon juice, paprika, cayenne pepper, and crushed red pepper flakes in a small bowl.
Remove 3 tablespoons of the sauce, reserving it for later.
Place the chicken breasts into a shallow pan. Pour the marinade onto the chicken breasts, then toss to coat evenly. Place into the refrigerator to marinate for 2-3 hours.
Heat a grill pan coated with cooking spray over medium heat.
Place the chicken on the hot grill pan and cook for 6-7 minutes.
Flip and continue grilling for 6-7 minutes, or until a meat thermometer reads 165 degrees.
Let the chicken rest on a cutting board for a few minutes before serving.
Drizzle the reserved sauce over the chicken or serve it on the side. Enjoy.