Marinate the Chicken: In a bowl, mix together the 1 ½ lbs boneless, skinless chicken breast with 1 tbsp soy sauce, 1 tsp cornstarch, ½ tsp black pepper, and ½ tsp garlic powder. Let the mixture marinate for 10-15 minutes while you prepare the other ingredients. The cornstarch will create a velvety texture, and help the sauce cling to the chicken.
Prepare the Sauce: In a small bowl, whisk together 2 tbsp soy sauce, 1 tbsp oyster sauce, 1 tbsp honey (or brown sugar), ½ tsp sesame oil, ½ cup chicken broth, 1 tsp freshly ground black pepper, and 1 tsp cornstarch (for thickening). Set this mixture aside; it will act as the flavorful black pepper sauce that brings everything together.
Sear the Chicken: Heat 1 tbsp vegetable oil in a large skillet or wok over medium-high heat. Add the marinated chicken pieces and stir-fry for 3-4 minutes, until they turn golden brown and are fully cooked. Once done, remove the chicken from the pan and set it aside.
Stir-Fry the Vegetables: Add another tablespoon of vegetable oil to the pan. Sauté the 3 cloves of minced garlic, 1-inch piece of grated ginger, and 1 medium sliced onion for 1-2 minutes, until fragrant. Stir in the 1 ½ cups of sliced mushrooms and the optional ½ red and ½ green bell peppers and cook for an additional 2-3 minutes, stirring frequently.
Combine Everything: Return the cooked chicken to the pan. Pour in the prepared black pepper sauce and mix well, ensuring the sauce coats all the ingredients. Stir-fry for another 2 minutes, allowing the sauce to thicken slightly.
Garnish and Serve: Remove the pan from heat and sprinkle chopped green onions and sesame seeds (if using) on top. Serve the flavorful Black Pepper Chicken hot, over a bed of steamed rice or noodles, and enjoy!