Toss sliced chicken with cornstarch, salt, and black pepper. Set aside.
In a small bowl, whisk together soy sauce, oyster sauce, broth, sugar, and black pepper.
Heat oil in a wok or large skillet over high heat. Add chicken and sear until just cooked through. Remove and set aside.
Add a bit more oil if needed. Stir fry onion and bell peppers for 2–3 minutes until crisp-tender.
Add garlic and ginger; cook until fragrant (about 30 seconds).
Return chicken to the pan. Pour in sauce and toss until thickened and glossy.
Remove from heat and serve immediately.