Preheat and prep the pan. Heat the oven to 425°F (220°C). Line a large rimmed sheet pan with parchment for easy cleanup.
Make the sauce. In a small bowl, whisk pineapple juice, soy sauce, rice vinegar, honey, sesame oil, ginger, garlic, cornstarch (if using), and red pepper flakes until smooth.
Toss the chicken. Add chicken pieces to a medium bowl.
Spoon over about half the sauce and toss to coat. Set aside to briefly marinate while you prep the veggies.
Prep the veggies and pineapple. On the sheet pan, combine bell peppers, red onion, broccoli, and pineapple. Drizzle with oil, then sprinkle with salt and pepper.
Toss directly on the pan to coat and spread into an even layer, leaving some space for the chicken.
Add the chicken. Nestle the marinated chicken pieces among the veggies and pineapple in a single layer. Discard any leftover marinade from the chicken bowl.
Roast. Place the pan on the center rack and roast for 15 minutes. Remove, toss everything gently, then brush or drizzle with half of the remaining sauce.
Finish roasting. Return to the oven and roast another 8–10 minutes, until the chicken is cooked through and the edges of the veggies start to char.
Internal temperature should reach 165°F (74°C).
Glaze and rest. Brush on the last of the sauce and let everything rest on the pan for 3 minutes. This helps the juices settle and the glaze cling.
Serve. Spoon over rice, quinoa, or cauliflower rice. Top with cilantro or green onions, a squeeze of lime, and sesame seeds.