Place the chicken legs in a large saucepan. Pour in 2l water and bring to the boil. Simmer over a very low heat.
Peel and halve 4 onions.
Peel the ginger and cut into slices.
Add the onions and ginger to the chicken and continue simmering. Skim off the foam from the surface.
Add the garlic, mustard seeds, coriander and cloves to the pan and simmer for another 30 minutes. Remove the chicken from the pan and leave to cool.
Pull the meat from the bones and remove the skin. Place the bones and leftovers back into the pot, boil again and simmer, covered, for 11/2 hours. Tear or cut the meat into small pieces.
Drain the soup into another pan through a sieve lined with a muslin cloth. Bring to boil again, then season with salt, sugar and fish sauce.
Peel the remaining onion and cut into fine rings. Place in a bowl of water until ready to serve.
Cut the meat into pieces.
Divide the rice between bowls.
Wash and slice the spring onions.
Boil the soup again. Add the spring onions, drained onion, chicken and some coriander to the rice. Pour over the hot soup and serve.