Preheat oven to 400°F (200°C). Line a large rimmed baking sheet with parchment paper or lightly grease with cooking spray.
In a large bowl, combine cubed chicken, bell peppers, and red onion. Drizzle with olive oil, then add minced garlic, salt, black pepper, paprika, and chili flakes. Toss until evenly coated.
Spread the chicken and vegetable mixture in a single layer on your prepared baking sheet. Keep the pineapple separate for now.
Roast for 20 minutes, flipping halfway through for even browning.
While roasting, whisk together soy sauce, pineapple juice, honey, and rice vinegar in a small saucepan. Simmer for 3-4 minutes until slightly thickened. For glossier glaze, stir in cornstarch slurry and cook another minute.
After 20 minutes, remove pan from oven and scatter pineapple chunks over chicken and vegetables. Pour half the glaze over everything and toss gently.
Return to oven for 5-8 more minutes until chicken reaches internal temperature of 165°F (74°C) and pineapple edges look golden.
Drizzle remaining glaze over finished dish. Garnish with fresh cilantro and sesame seeds before serving.