Marinate chicken in soy sauce, brown sugar, rice vinegar, sesame oil, ground ginger, garlic powder, and optional red pepper flakes for at least 30 minutes.
Preheat oven to 400°F (200°C). Chop the bell peppers, onion, broccoli, and halve cherry tomatoes. Drain pineapple chunks and reserve juice.
On a lined baking sheet, combine veggies and pineapple; top with marinated chicken.
Bake for 25-30 minutes until chicken reaches an internal temperature of 165°F (74°C), tossing halfway through.
While baking, whisk reserved pineapple juice with cornstarch, soy sauce, honey, and rice vinegar in a saucepan until thickened.
Drizzle sauce over chicken and veggies before serving with warm cooked rice.