Step 1: Preheat your grill to medium-high heat (about 400°F/200°C) or your oven to 400°F (200°C).
Step 2: In a large bowl, combine the diced chicken pieces, teriyaki sauce, and minced garlic. Stir until the chicken is well-coated. Let it marinate for at least 15 minutes while you prepare other ingredients.
Step 3: Tear off four large pieces of heavy-duty aluminum foil (approximately 12×12 inches each).
Step 4: Divide the marinated chicken evenly among the centers of the four foil pieces.
Step 5: Top the chicken in each packet with an equal amount of diced pineapple, sliced bell pepper, and sliced onion.
Step 6: Drizzle a small amount of olive oil over the vegetables in each packet and season with salt and pepper to taste.
Step 7: Bring the long sides of each foil piece together over the food and fold them down several times to create a tight seam. Then, fold in the short ends tightly to completely seal the packet. Ensure no gaps for steam to escape.
Step 8: Place the sealed foil packets directly on the preheated grill grates or on a baking sheet in the oven.
Step 9: Cook for 15-20 minutes, or until the chicken is cooked through and reaches an internal temperature of 165°F (75°C).
Step 10: Carefully remove the packets from the heat. Let them rest for 1-2 minutes before cautiously opening (watch out for hot steam).
Step 11: Serve the contents of the packets directly or transfer to plates. Garnish with sesame seeds, sliced green onions, or cilantro if desired, and enjoy!