Slice the flank steak thinly, cutting against the grain into ¼-inch-thick strips. Transfer the slices to a large bowl.
Add the marinade ingredients — water, cornstarch, soy sauce, baking soda, and oil — to the beef. Mix well until the slices are evenly coated. Let the beef rest for about 15 minutes at room temperature to absorb the flavors. Avoid marinating for longer than 30 minutes to preserve the texture and taste.
Meanwhile, in a small mixing bowl, whisk together the sauce ingredients: chicken stock, oyster sauce, dark soy sauce, Shaoxing wine, cornstarch, sugar, sesame oil, and minced garlic. Set the sauce aside.
Heat a tablespoon of oil in a large wok or skillet over medium-high heat. Once the oil is shimmering, add the marinated beef in a single layer. Stir-fry until browned and just cooked through, about 4 to 5 minutes. Remove the beef from the pan and set aside.
In the same pan, toss in the sliced onions and quartered mushrooms. Stir-fry for about 2 minutes until the onions begin to soften and the mushrooms release their moisture. Pour in the prepared sauce and stir continuously until it starts to thicken and coat the vegetables.
Return the cooked beef to the pan, followed by the green onions. Stir everything together for another 30 seconds, ensuring the beef is evenly coated with the rich, glossy sauce.
Serve immediately with steamed rice or noodles, and enjoy the savory harmony of tender beef and earthy mushrooms.