Toss chicken with cornstarch and a pinch of salt until evenly coated.
Heat vegetable oil in a large skillet or wok over medium-high heat. Cook chicken for 6–7 minutes until golden and cooked through. Remove and set aside.
In the same pan, sauté onion and bell pepper (if using) for 2–3 minutes.
In a bowl, whisk together all sauce ingredients except the slurry.
Pour the sauce into the skillet. Bring to a simmer, then stir in the cornstarch slurry and cook for 1–2 minutes until thickened.
Return chicken to the skillet and toss to coat in the sauce.
Garnish with green onions or sesame seeds and serve hot over rice.