Preheat the oven to 425 degrees F. Generously coat a half sheet pan (or 2 large baking sheets) with nonstick cooking spray.
In a large bowl, whisk together the soy sauce, brown sugar, vegetable oil, sesame oil, salt, pepper, garlic powder, and red pepper flakes.
Add in the chicken, broccoli, carrots, and onion. Toss to thoroughly coat everything.
Place the chicken pieces to one side of the baking sheet, spreading them apart so they don’t touch. Scatter the vegetables in a single layer on the other side.
Place in the oven and roast for about 20-25 minutes until the chicken is cooked through (and has an internal temperature of 165 degrees F.) and vegetables are tender.
Mix everything together and serve over cooked white rice with an extra drizzle of soy sauce, sesame oil, and a sprinkle of sesame seeds.