Step 1: Marinate the Chicken

In a bowl, mix the chicken pieces with cornstarch, soy sauce, salt, and black pepper. Let it marinate for at least 15 minutes while you prepare the other ingredients.

Step 2: Prepare the Sauce

In a small bowl, whisk together oyster sauce, soy sauce, dark soy sauce, hoisin sauce, rice vinegar, brown sugar, black pepper, garlic, ginger, red chili flakes, and water. Set aside.

Step 3: Cook the Chicken

Heat 1 tablespoon of vegetable oil in a large pan or wok over medium-high heat. Add the marinated chicken and stir-fry for about 5-6 minutes until golden brown and fully cooked. Remove from the pan and set aside.

Step 4: Stir-Fry the Vegetables

In the same pan, add another tablespoon of vegetable oil. Add the sliced onions and bell peppers, stir-frying for 2-3 minutes until slightly softened.

Step 5: Combine Everything

Return the cooked chicken to the pan with the vegetables. Pour in the prepared sauce and mix well. Let it cook for 2-3 minutes until the sauce thickens slightly. Add the cornstarch slurry to thicken the sauce further.

Step 6: Finish with Sesame Oil & Green Onions

Drizzle sesame oil over the dish and sprinkle chopped green onions. Give it a final stir and turn off the heat.