Marinate the Chicken: In a bowl, combine sliced chicken with soy sauce, cornstarch, and sesame oil. Let it sit for 10 minutes while you prepare other ingredients.
Make the Sauce: In a small bowl, whisk together oyster sauce, soy sauce, rice vinegar, sugar, black pepper, and water. Set aside.
Sear the Chicken: Heat 1 tablespoon oil in a large skillet or wok over medium-high heat. Add the marinated chicken in a single layer. Cook for 3–4 minutes on each side or until fully cooked and lightly browned. Remove from the pan and set aside.
Stir-Fry the Vegetables: In the same skillet, add the remaining 1 tablespoon oil. Sauté garlic and ginger for 30 seconds. Add onions and bell peppers and stir-fry for 2–3 minutes until just tender but still crisp.
Combine Everything: Return the cooked chicken to the pan, pour in the sauce, and toss everything together. Let it simmer for another 2 minutes until the sauce thickens slightly and evenly coats the chicken and veggies.
Serve: Garnish with chopped green onions and serve hot with steamed jasmine rice or noodles.