Pour 1/4 cup Wish-Bone® Italian Dressing over chicken in large, shallow nonaluminum baking dish or plastic bag. Cover, or close bag, and marinate in refrigerator, turning occasionally, 30 minutes to 3 hours.
Meanwhile, combine tomatoes, onion, basil and 1/4 cup Dressing in medium bowl. Cover and marinate in refrigerator at least 30 minutes.
Remove chicken from marinade, discarding marinade. Grill or broil chicken, turning once and brushing frequently with remaining 1/4 cup Dressing, until chicken is thoroughly cooked, about 12 minutes. Serve tomato mixture over chicken.