- Bring a teakettle of water to a boil. Put the dried mushrooms in a small bowl, cover them with boiling water, and let them soak for 15 minutes. Drain the mushrooms, reserving 1/3 cup of the soaking liquid. Discard the tough stems from the mushrooms, slice the caps very thin, and set them aside. Whisk the cornstarch with the soy sauce, vinegar, oyster sauce, and the reserved mushroom soaking liquid in a small bowl.
- Heat the oil in a wok or large skillet over high heat. Add the garlic and ginger and stir-fry for 30 seconds. Add the beef and stir-fry just until it is no longer pink. Push it to the outer perimeters of the pan and add the stems of the bok choy to the center where the heat is concentrated. Stir continuously until the bok choy is crisp-tender, about 3 minutes. Add the bok choy leaves, reserved mushroom caps, and chiles, if using, and stir for one minute. Add the soy sauce mixture and stir until the sauce is bubbling and slicks the greens and beef well. Serve immediately.