Slice some spring onions to garnish later and place the remainder in a large saucepan with the ginger, star anise, cinnamon, lemongrass and stock. Simmer for 20 minutes. Add the fish sauce, sugar and lime juice to taste.
Meanwhile, place the noodles in a heatproof bowl, cover with boiling water and cover the bowl with clingfilm, leave to soak for 10 minutes or until tender.
Heat a frying pan until very hot and cook the steak for 2 minutes on each side until nicely browned but pink in the centre. Transfer to a plate and leave to rest for few minutes. If you are making the vegetarian version then slice mushrooms and stir fry for a couple of minutes.
Divide the beansprouts, pak choi, rice noodles between four bowls. Ladle over the broth and vegetables then thinly slice the steak and place on top. Scatter with reserved spring onions, chilli, Thai basil and coriander leaves, and serve with lime wedges on the side.