For marinade, combine Wish-Bone® Italian Dressing, tomatoes and 2 tablespoons cheese. Pour 1/2 cup marinade over chicken in large, shallow nonaluminum baking dish or plastic bag; turn to coat. Cover, or close bag, and marinate in refrigerator, turning occasionally, up to 3 hours. Refrigerate remaining marinade.
Remove chicken from marinade, discarding marinade. Grill or broil chicken, turning once and brushing frequently with 1/4 cup refrigerated marinade, until chicken is thoroughly cooked. To serve, slice chicken. Arrange chicken, mushrooms and onion over spinach; drizzle with remaining refrigerated marinade. Sprinkle with remaining 2 tablespoons cheese and, if desired, cracked black pepper.