Wash lemon grass, dab dry and cut or chop into fine rings. Wash lemon leaves. Wash the chillies, dab dry, cut lengthwise and remove the seeds. Cut the chillies into fine rings.
Squeeze the limes. Bring coconut milk, some salt, lemon grass and leaves to the boil in a pot. Let it simmer at low heat for about 5 minutes. In the meantime clean the mushrooms and cut them into thin slices, except for about 75 g.
Pour the coconut milk through a fine sieve, return to the pot and bring to the boil. Stir in half the chilli rings, lime juice and fish sauce. Add mushroom slices and let everything simmer for 3-4 minutes.
Meanwhile wash the coriander green, dab dry and pluck off the leaves. Leave the leaves whole or cut them into strips as desired. Slice the remaining mushrooms very thinly. Season the soup with fish sauce or a little salt and serve in bowls.
Serve sprinkled with sliced mushrooms, chilli rings and coriander leaves.