In a small bowl, whisk together all sauce ingredients until smooth.
Toss sliced chicken with cornstarch if using.
Heat oil in a large skillet or wok over medium-high heat.
Add chicken in a single layer and cook until golden and slightly crispy. Remove and set aside.
In the same pan, add vegetables and stir fry 2–3 minutes until tender-crisp.
Return chicken to the pan and pour in the sauce.
Toss everything together and simmer 1–2 minutes until the sauce thickens and coats the chicken.
Garnish if desired and serve immediately.