In a large bowl, whisk together jerk seasoning, 2 tablespoons cider vinegar, and olive oil. Add boneless, skinless chicken breasts, and turn to coat. Cover, and let marinate in the refrigerator 2 hours.
Preheat oven to 375 degrees. Remove chicken from marinade, and place on a rimmed baking sheet. Spoon marinade over chicken, and bake until chicken
is just cooked through, about 25 minutes.
Meanwhile, in a medium bowl, toss onion and cucumber with 3 tablespoons cider vinegar and sugar. To serve, slice chicken breasts as desired; spoon cucumber relish over the top.