Set aside some spring onion slices to garnish and place the remainder in a large saucepan with the ginger, star anise, lemongrass and stock and simmer for 10 minutes.
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.
Place the beansprouts and pak choi in a large saucepan and pour or strain over the hot stock. Bring to a simmer, add the fish sauce, sugar and lime juice to taste.
Drain the noodles and divide between bowls. Ladle over the broth and vegetables then thinly slice the steak and place on top. Scatter with reserved salad onions, chilli and serve with lime wedges on the side