Preheat oven to 400°F (200°C). Line a large rimmed sheet pan with parchment paper or foil and lightly coat with oil.
In a medium bowl, whisk together orange juice, zest, soy sauce, honey, garlic, ginger, vinegar, and red pepper flakes. Dissolve 1 tablespoon cornstarch in 2 tablespoons water and whisk into sauce. Set aside.
Pat chicken dry with paper towels. In a large bowl, toss chicken with 1.5 tablespoons olive oil, garlic powder, salt, and pepper. Add 2 tablespoons cornstarch and toss to coat.
Pour half the orange sauce over chicken and toss to coat. Reserve remaining half for later.
In the same bowl, toss broccoli, bell peppers, and red onion with remaining 1.5 tablespoons olive oil, salt, and pepper. Spread vegetables around edges of the sheet pan, leaving center open.
Place chicken pieces in a single layer in the center of the pan. Pour any remaining sauce from the bowl over the chicken.
Roast for 20-25 minutes, until chicken reaches 165°F (74°C) and vegetables have golden edges. For extra caramelization, broil for final 2-3 minutes.
While chicken roasts, heat reserved sauce in a small saucepan over medium heat until boiling and thickened, about 3-4 minutes.
Pour thickened sauce over roasted chicken and vegetables. Toss gently to coat everything evenly.
Garnish with sliced green onions and sesame seeds. Serve immediately over steamed rice, spooning extra sauce from the pan over each bowl.