Whisk together soy sauce, oyster sauce, Shaoxing wine, and sugar in a small bowl until the sugar dissolves.
In a separate small container, whisk cornstarch and cold water to create a slurry.
Heat 1 tablespoon of oil in a wok over high heat until it reaches its smoke point.
Add sliced mushrooms and sear until browned and moisture has evaporated, then remove and set aside.
Add the remaining tablespoon of oil to the wok. Add minced garlic and half of the cracked black pepper, sautéing for 30 seconds until aromatic.
Add chicken pieces in a single layer and sear for 2 minutes without moving, then stir-fry until 90 percent cooked.
Return the mushrooms to the wok and pour the prepared sauce over the mixture.
Stir in the cornstarch slurry and the remaining black pepper, tossing constantly for 60 seconds until the sauce thickens and coats all components.
Garnish with green onions and serve immediately over steamed rice.