To make the meat extra tender and flavorful, it’s important to marinate it first. In a medium bowl, stir together the cornstarch, 2 tablespoons of the soy sauce, and the rice wine until the cornstarch is dissolved. Add the beef and toss to coat. Let stand for at least 10 minutes. While the steak is marinating this is the perfect time to prep the rest of the ingredients.
Heat a wok or large skillet over high heat until a bead of water sizzles and evaporates on contact. Add the oil and swirl to coat the bottom. Add the beef and spread it out in a single layer. Allow it to sear untouched for 1 minute, then stir-fry for another 1 to 2 minutes until the pink is mostly gone. Transfer to a plate and set aside.
In the same wok or skillet, add the garlic and stir-fry very briefly (about 20 seconds), just until aromatic. Add the green beans and stir-fry for another 1 minute. Add the water and immediately cover with a lid. Allow the green beans to steam for 3 to 4 minutes, or until they are crisp-tender and the water has evaporated.
Return the beef to the pan and also add the mushrooms and bok choy. Add the fish sauce, the remaining 3 tablespoons of soy sauce, and the black pepper. Stir so that everything is well coated, then turn off the heat. Transfer to a plate and serve.