Marinate the Chicken: In a bowl, toss sliced chicken with soy sauce and cornstarch. Let it marinate for 10 minutes while preparing other ingredients.
Prepare the Black Pepper Sauce: In a small bowl, whisk together soy sauce, oyster sauce, rice vinegar, sugar, chicken broth, and black pepper. Set aside.
Sear the Chicken: Heat 1 tbsp oil in a large pan or wok over high heat. Add chicken in a single layer and let it sear for 30 seconds before stirring. Cook for 2–3 minutes until browned, then remove from the pan.
Sauté the Aromatics: Add the remaining 1 tbsp oil to the pan. Sauté garlic and ginger for 30 seconds until fragrant.
Stir-Fry the Vegetables: Add bell peppers and onions, cooking for 2 minutes until slightly softened but still crisp.
Add Sauce & Thicken: Pour in the black pepper sauce, stirring quickly. If a thicker sauce is desired, add cornstarch slurry and mix until the sauce coats everything evenly.
Combine & Serve: Return the cooked chicken to the pan, tossing everything together until well-coated in sauce. Cook for another minute.
Garnish & Serve: Sprinkle with green onions and sesame seeds. Serve hot over steamed rice.