For the Tom Yam Gung, peel the shrimp to the tail. Soak the mushrooms, cut them in half, drain well. Bring water to a boil, add lemongrass and kaffir leaves, season with salt and simmer on low heat for 30 minutes.
Remove soup from heat, season with prik nam pla and lemon juice. Cook the crayfish tails, mushrooms with coriander leaves in the soup for another 2-3 minutes.
Then bring the Tom Yam Gung hot to the table.