Marinate the chicken with soy sauce, oyster sauce, cornstarch, and black pepper for 10 minutes.
Whisk together soy sauce, oyster or hoisin sauce, brown sugar, white pepper, cornstarch slurry, and chicken broth to make the sauce.
Heat oil in a wok or skillet over medium-high heat, then sear the chicken until nearly cooked through; remove and set aside.
Add onion, bell peppers, garlic, and ginger to the same pan; stir-fry for 3–4 minutes until tender-crisp.
Return the chicken to the pan, pour in the sauce, and cook until thickened and glossy.
Finish with sesame oil and more cracked black pepper. Garnish with sliced green onions and serve hot over rice.