In a bowl, toss sliced chicken with cornstarch, salt, and black pepper.
Heat oil in a large skillet over medium-high heat. Add chicken and cook until golden and cooked through. Remove and set aside.
In the same skillet, add bell peppers and sauté for 2–3 minutes until tender-crisp.
Add garlic and cook for 30 seconds until fragrant.
Whisk sauce ingredients together and pour into the skillet.
Return chicken to the pan and toss until sauce thickens and coats everything evenly.
Garnish if desired and serve immediately.