Add chicken and cornstarch to a large ziplock bag. Shake to coat the chicken with the cornstarch.Meanwhile heat 1 tablespoon of oil in a large skillet.
Working in two batches add the chicken and cook over medium high heat until browned, 4-5 minutes.Remove the chicken from the pan and set aside.
Add the remaining tablespoon of oil to the same pan.Add the peppers and saute until browned and slightly tender, 3-4 minutes.
Meanwhile prepare the sauce by whisking together sugar, vinegar, ketchup, soy sauce, onion and garlic powder in a small bowl.
Add the sauce and the chicken back to the pan.
In a small bowl whisk together 1 tablespoon of cornstarch with 2 tablespoons of water.Slowly pour the cornstarch mixture into the sauce. Bring to a simmer and cook until thickened, 2-3 minutes.
Serve immediately with green onions and sesame seeds for garnish, if desired.