Lightly coat chicken and onion with oil, then season, if desired, with salt and ground black pepper. Grill chicken, onion and peppers, turning once, 8 minutes or until chicken is thoroughly cooked and vegetables are tender. Wrap peppers in aluminum foil; let stand 5 minutes, then remove skin and thinly slice. Thinly slice chicken; set aside.
Grill top side of pizza crusts about 1 minute; remove from grill. Evenly spread pizzas with pasta sauce, then evenly top with chicken, vegetables and cheese. Grill pizzas, covered, until cheese is melted, about 3 minutes.
Sprinkle with cilantro.
Serve, if desired, with additional heated sauce and diced avocado.