Toss sliced chicken with cornstarch until evenly coated.
Heat 1 tablespoon oil in a large skillet over medium-high heat. Add chicken and cook until lightly golden and cooked through. Remove and set aside.
Add remaining oil to the skillet. Stir fry peppers and onion for 3–4 minutes until tender-crisp.
Add garlic and cook for 30 seconds until fragrant.
Whisk sauce ingredients together and pour into the skillet. Simmer until slightly thickened.
Return chicken to the pan and toss until evenly coated and heated through.
Serve immediately over rice or noodles.