Lightly coat a 9×13 baking dish with nonstick spray.
To make the teriyaki sauce: add all the sauce ingredients except cornstarch, to a small saucepan over medium heat, whisk to combine and bring it to a simmer.
Mix cornstarch with 1 tablespoon of water until fully combined. Stir in cornstarch mixture into the sauce, and simmer until thickened enough to coat the back of a spoon, about 2 minutes. Let the sauce cool to room temperature. Reserve 1/2 cup and set aside.
Add chicken to a gallon size Ziploc bag or to a large tupperware bowl. Add chicken and add the remaining sauce. Marinate for at least 30 minutes or overnight, turning the bag occasionally to make sure the chicken marinates uniformly.
When ready to cook, drain the chicken from the sauce. Do not discard the sauce yet.
Add the chopped veggies to a large bowl, add the olive oil, red pepper flakes and salt. Stir to combine.
Add the veggies to the prepared baking pan. Top with the chicken and the remaining teriyaki sauce.
Place into oven and roast until completely cooked, about 25-30 minutes. With an internal temperature of 165 degrees F.
Check the chicken and turn on the broiler for the last 5 minutes, for a crispier top on the chicken.
Serve immediately over rice if desired, with veggies on the side, garnished with sesame seeds and green onion.