Place the chicken chunks into a bowl with the water, soy sauce, oyster sauce, vegetable oil, and white pepper. Mix until thoroughly combined and set aside for 30 minutes.
After 30 minutes, mix the cornstarch into the marinated chicken, massaging everything together until the chicken has absorbed any standing liquid.
Meanwhile, mix the sauce by combining the chicken stock, oyster sauce, soy sauce, sesame oil, sugar, and white pepper. Set aside.
Place a clean, dry wok over medium high heat, and add 2 tablespoons of oil and the smashed ginger. After 15 seconds, add the pickled mustard greens, and stir-fry for 1 minute, until the greens are seared and slightly dry.
Next, add the smashed garlic, white parts of the scallions, and dried red peppers (if using). Stir-fry for another 30 seconds. Searing the aromatics and the mustard greens like this really brings out their flavors.
Move the mustard greens to the sides of the wok, and turn the heat up to the highest setting. Add the remaining tablespoon of oil to the open middle area of the wok, and add the chicken in 1 layer. Let the chicken fry for 1 minute on each side, or until lightly browned. Move the mustard greens and aromatics around with the chicken so they don’t burn.
Add the Shaoxing wine around the perimeter of the wok and stir everything together for 10 seconds.
Add prepared sauce mixture to deglaze the wok, mixing everything together until combined. Then push all of the ingredients to the middle of the wok. Turn the heat down so the liquid is at a slow simmer.
Cover and cook for 3-4 minutes, or until the sauce has nearly completely reduced. If it looks too dry, add a little more stock. If it’s still too wet, simmer a bit longer with the cover off until it has reduced to your liking.
Turn the heat back up to medium high heat and add the green parts of the scallions. Add enough of the cornstarch slurry to thicken the sauce (you don’t have to use it all) and stir together for 20-30 seconds. Plate and serve with steamed rice.