GRILL DIRECTIONS: In medium bowl, combine soy sauce, reserved 3 tablespoons pineapple liquid, honey, 2 teaspoons of the sesame oil, garlic powder and ginger; mix well. Set aside.
Place 1 chicken breast half between 2 pieces of plastic wrap or waxed paper. Working from center, gently pound with flat side of mallet or rolling pin until about 1/4 inch thick; remove wrap. Repeat with remaining chicken breast halves.
Add chicken to soy sauce mixture; turn to coat. Cover; let stand at room temperature for 15 minutes to marinate.
Heat grill. In small bowl, combine onions, mayonnaise, chutney and remaining 1/4 teaspoon sesame oil. Set aside.
When ready to grill, remove chicken from marinade; place pineapple slices in marinade. Place chicken on gas grill over medium heat or on charcoal grill 4 to 6 inches from medium coals. Cook 6 to 8 minutes or until chicken is fork-tender and juices run clear, turning once.
Remove pineapple from marinade; discard marinade. Place pineapple on grill; cook about 2 minutes or until browned, turning once. Place bread slices on grill; cook about 1 minute or until toasted, turning once.
Spread each slice of toasted bread with about 2 tablespoons mayonnaise mixture; cut bread slices in half crosswise. Place chicken on halves of bread. Top with pineapple slices and other halves of bread.