In a large bowl, combine the cut chicken pieces with 1 tbsp soy sauce, 1 tbsp cornstarch, 1 tsp sesame oil, and 1 tsp freshly ground black pepper. Mix well to ensure every piece is coated. Let marinate at room temperature for at least 15 minutes, or refrigerate for up to 30 minutes.

While the chicken marinates, prepare the sauce. In a small bowl, whisk together 2 tbsp soy sauce, 1 tbsp oyster sauce, 1 tsp granulated sugar, 1 tsp rice vinegar, 1/2 cup chicken broth, 1 tbsp cornstarch, and 2-3 tsp freshly ground black pepper. Whisk until the cornstarch is fully dissolved and the sauce is smooth. Set aside.

Heat 2 tbsp cooking oil in a large wok or skillet over medium-high heat until shimmering. Add the marinated chicken in a single layer (cook in batches if necessary to avoid overcrowding, which steams the chicken instead of browning). Cook for 3-5 minutes, flipping occasionally, until the chicken is browned and mostly cooked through. Remove the chicken from the wok and set aside on a clean plate.

If needed, add another teaspoon of oil to the wok. Add the minced ginger and garlic and stir-fry for 30 seconds until fragrant. Add the bell pepper and yellow onion pieces and stir-fry for 2-3 minutes until they are tender-crisp.

Return the cooked chicken to the wok with the vegetables. Give the prepared Black Pepper Sauce a quick re-whisk (cornstarch can settle at the bottom) and pour it over the chicken and vegetables. Stir continuously for 1-2 minutes until the sauce thickens and coats all the ingredients beautifully.

Remove from heat. Garnish generously with sliced green onions. Serve immediately with steamed white rice or noodles.