Cover with chicken stock and add the kaffir lime leaves. Simmer gently for 10 minutes.
Season the broth with the fish sauce and palm sugar and adjust the flavour with the lime juice.
Return the pan to the heat and bring the broth to a simmer, add the prawns and straw mushrooms and simmer gently for a few minutes, until the prawns are just cooked.
Sprinkle with kaffir lime leaf chiffonade and coriander leaves to garnish.