In medium bowl, mix relish ingredients; set aside. Heat gas or charcoal grill. Coat both sides of each chicken breast evenly with Cajun seasoning.
When grill is heated, carefully oil grill rack. Place chicken on gas grill over medium heat or on charcoal grill over medium coals; cover grill. Cook 10 minutes, turning once, until juice of chicken is no longer pink when center of thickest part is cut (170°F). Serve chicken with relish.