Lay out four 12x12 squares of heavy duty aluminum foil (or if using regular foil, use two layers). Spray the foil with non-stick spray.
Season the chicken with salt and pepper. Coat the chicken in 1/2 cup of teriyaki sauce. Place one piece of chicken on each square of foil.
To a mixing bowl, add the bell peppers, pineapple, red onions, and snap peas. Toss with 1/2 cup teriyaki sauce to coat. Season with salt and pepper. Evenly distribute the vegetables and pineapple onto each of the pieces of chicken.
Bring the sides of the foil up and fold them over tightly, ensuring that each sheet forms an enclosed packet over the food.
If using a grill: preheat grill to medium with the lid closed. Place the packets onto the grill, close the lid, and cook 15-20 minutes (or until the vegetables are tender crisp and the chicken is completely cooked through), turning over once halfway through cooking.
If using an oven: preheat oven to 425 degrees. Place the packets into the oven and bake 20-25 minutes until the vegetables are tender crisp and the chicken is cooked completely in the center.
The foil packets will be very hot; take caution when opening and releasing the steam. Open the packets and sprinkle each one with scallions and sesame seeds. Serve with additional teriyaki sauce.