Marinate the Chicken: In a bowl, combine chicken strips with soy sauce, oyster sauce, cornstarch, sesame oil, and black pepper. Refrigerate for 20–60 minutes.
Prepare the Sauce: Whisk together all sauce ingredients in a small bowl; set aside.
Heat the Wok: Over high heat, add 1 tbsp oil. When shimmering, add chicken in a single layer. Sear undisturbed for 2–3 minutes, then stir‑fry until almost cooked through (2–3 more minutes). Transfer to a plate.
Cook the Aromatics & Vegetables: Add remaining oil, then sauté garlic and ginger for 30 seconds until fragrant. Add onion and bell peppers; stir‑fry 2–3 minutes until just tender‑crisp.
Combine & Finish: Return chicken to the wok. Stir sauce to recombine, pour over contents, and toss continuously until sauce thickens (~2 minutes). Stir in scallions, then remove from heat.
Serve Immediately: Plate over steamed rice, noodles, or cauliflower rice.