Heat the oil in a large pan and add the mushrooms and chilli. Cook for 2 minutes, stirring. Add the ginger and cook for a further few seconds. Pour in the stock and bring to a simmer. Add the pak choi, pork and noodles. Bring back to a simmer and cook for 5 minutes.
Mix the miso paste with a little hot stock from the pan. Stir into the cooked broth along with the coriander. Serve with lime wedges.