Preheat the oven to 400°F. Line a sheet pan with parchment paper.
In a large bowl, toss together the pineapple, red pepper, onion, oil, garlic powder, salt, and black pepper and mix well. Transfer to the prepared sheet pan.
Add the cubed chicken and ¾ cup of the barbecue sauce to the same bowl. Toss to evenly coat. Transfer to the sheet pan, spreading everything out into one even layer.
Bake until the pineapple and bell pepper are softened and the chicken reaches an internal temperature of 165°F (73°C) in 13 minutes.
To assemble, arrange 3 or 4 lettuce leaves on each of four plates. Divide the chicken and pineapple mixture over the lettuce leaves, drizzle with the remaining ¼ cup barbecue sauce, and garnish with sliced green onions.
