Toss chicken pieces with cornstarch until well coated.
In a large skillet or wok over medium-high heat, heat oil. Add chicken and sear until golden brown and cooked through, about 5–6 minutes. Remove and set aside.
In the same pan, add a bit more oil if needed. Stir-fry onion, bell peppers, garlic, and ginger for 2–3 minutes until slightly softened but still crisp.
In a small bowl, whisk together soy sauce, oyster sauce, hoisin, rice vinegar, brown sugar, black pepper, and water. Pour the sauce into the skillet with the veggies.
Return the chicken to the pan and toss to coat. Simmer for 3–4 minutes until the sauce thickens and clings to the chicken.
Serve over cooked rice. Garnish with green onions or sesame seeds if desired.