Soak noodles in warm water 5 minutes to soften, then drain.
Place stock, lemongrass and ginger into a saucepan and bring to a simmer. Simmer for 5 minutes then strain. Return the stock to the pan and add fish sauce to taste.
Add the chicken and bok choy and simmer 3 minutes or until chicken is cooked. Add spring onion, chilli, lime juice and coriander. Finally add the noodles, heat through and serve.