Heat the oil in a wok over a high heat until the oil starts to shimmer. Add the pak choi, a handful at a time, stirring occasionally. Cover the pan and cook for about 2–3 minutes, until the pak choi leaves have wilted slightly, then remove to a serving plate.
Return the wok to the heat, add the mushrooms and stir-fry over a high heat for 30 seconds. Add the shoyu sauce, rice wine and the vegetable stock and stir to mix. Add the cornflour paste slowly, stirring constantly until the sauce has thickened.
Pour the mushrooms and sauce over the pak choi and serve immediately.