The essence of this soup is the clear fish liquor. It really is fantastically delicate and the flavours are multilayered and precise.
Put the crabs in a pan and cover with water. Add the ginger, lemongrass, kaffir lime leaves, shallot and a splash of fish sauce. Cover with a lid and bring to the boil. Simmer for 10-15 minutes. Remove the crabs and leave to cool.
Meanwhile, place the white fish in the hot stock that the crab was cooked in and leave to cook in its residual heat.
Pick the meat from the crabs. Reserve the white meat for the soup and use the dark meat for another dish.
Place the noodles in a serving bowl, then add the spring onions, crabmeat, chilli, rao ram herb and lime wedges.
Bring the broth back to the boil and ladle over the noodles, together with a piece of white fish.
Serve with fish sauce and freshly ground black pepper on the side.
