Season chicken with salt, pepper, and garlic powder. Add chicken and veggies to a large pan or non stick skillet and cook over medium heat 8-12 minutes, stirring throughout til chicken is cooked through.
In a medium bowl whisk together all sauce ingredients. Pour over chicken and veggies, and bring sauce to a boil. Whisk together cold water and corn starch. Pour cornstarch slurry into pan. Stir until sauce thickens. Serve warm with rice and garnish with green onions if desired.