In a large mixing bowl, whisk together the soy sauce, brown sugar, sesame oil, minced garlic, grated ginger, cornstarch, and water to create the teriyaki sauce.
Add the chicken breasts to the bowl and toss until well coated. Let marinate for at least 30 minutes, or up to 2 hours.
Preheat your oven to 400°F (200°C).
Prepare your vegetables by cutting the broccoli into florets, slicing the carrots, red onion, and bell pepper, then arrange in a single layer on a large sheet pan.
Once marinated, place the chicken on the sheet pan among the vegetables and drizzle with any remaining marinade. Season with salt and black pepper to taste.
Roast in the preheated oven for 25-30 minutes, or until the chicken reaches an internal temperature of 165°F (75°C) and the vegetables are tender.
Remove from the oven and let rest for a few minutes. Serve over rice or quinoa with additional teriyaki sauce if desired.
