Combine first 4 ingredients in a small bowl; stir will; and set aside.
Place chicken between 2 sheets of heavy duty plastic wrap, and flatten to 1/4-inch thickness,ยท using a meat mallet or rolling pin.
Preheat gas or charcoal grill with rack removed. With rack removed, coat rack with cooking spray; do not spray rack on grill while grill is lit, or else it will cause a fire!
Place oiled rack on grill over medium-hot coals or gas burners on medium heat. Allow to heat for 5 minutes. Place chicken on rack, and cook 4 minutes on each side, basting frequently with soy sauce mixture until chicken is cooked through. Transfer chicken to a serving platter; garnish with green onion, if desired. Makes 4 servings. Enjoy!