Place the stock into a large pot and add the spring onions

Place the ginger into a spice bomb and bring to the boil

Reduce the heat and simmer for an hour

Remove the spice bomb and add the lemongrass and lime leaves

Add back to the stock together with the bamboo shoots

Simmer for 10 minutes and then add 10mls fish sauce, the juice of half the lime, 2.5mls palm sugar and the chillies

Simmer for 5 minutes and then add the crayfish

Simmer for 5 minutes to cook the crayfish

Remove the spice bomb and add the tomatoes, the rest of the fish sauce, and the rest of the palm sugar if you think it's necessary

Remove from the heat and add the juice of the other half of the lime

Garnish with coriander and serve with [balachaung|http://tandysinclair.com/balachaung-dried-shrimp-relish-recipe/} if you want an enhanced shrimp paste flavour