Place the onion in a pot with the cut side down and roast for a few minutes.
Add the ginger and the chicken.
Fill with 1.5 litres water and bring to the boil.
Reduce the heat and simmer for around 90 minutes to 2 hours.
Scoop the foam off occasionally.
Soften the noodles in cold water for 30 min.
Remove the chicken from the soup and leave to cool.
Add the spices and the fish sauce to the soup, season with salt and simmer for a further 60 min.
Carve the chicken, removing the meat from the bones and cutting into bite-sized pieces.
Cook the noodles in boiling salt water for around 3 min then quench and drain well.
Sieve the soup through a cloth and divide the noodles and the meat into four bowls.
Pour the soup into the bowls and serve garnished with spring onion and chilli rings.