Slice and marinate the chicken. In a bowl, combine sliced chicken with 1 tablespoon soy sauce, Shaoxing wine, 1 teaspoon oyster sauce, sesame oil, 2 teaspoons cornstarch, and 1 tablespoon neutral oil. Toss well and let sit for 10–15 minutes while you prep the vegetables.
Make the sauce. In a measuring cup, whisk together black pepper, soy sauce, oyster sauce, dark soy (if using), rice vinegar, sugar, chicken broth, and 1–1.5 teaspoons cornstarch until smooth. Set aside.
Prep the aromatics and vegetables. Slice onion and peppers.
Mince garlic and ginger. Slice scallions, keeping whites and greens separate.
Heat the pan. Set a large wok or skillet over medium-high heat. Add 1 tablespoon oil and swirl to coat.
When it shimmers, add half the chicken in a single layer. Sear 2–3 minutes, flip, and cook another 1–2 minutes until just cooked. Remove to a plate.
Repeat with remaining chicken, adding oil if needed.
Stir-fry the vegetables. Add a touch more oil. Toss in onion, bell peppers, and scallion whites. Stir-fry 2–3 minutes until crisp-tender and lightly charred at the edges.
Add aromatics. Push veggies to the side.
Add garlic and ginger to the center with a small splash of oil. Stir 20–30 seconds until fragrant.
Combine chicken and sauce. Return chicken to the pan. Give the sauce a quick stir (cornstarch settles) and pour it in.
Stir-fry 1–2 minutes until the sauce thickens and coats everything evenly.
Finish and serve. Sprinkle in scallion greens. Taste and adjust salt or pepper. Serve hot over steamed jasmine rice or noodles.