Trim lemongrass stalks. Smash the stalks hard to bruise them as this releases flavour, then cut into 2 inch lengths. Slice galangal into 2mm slices. Smash red chillies. You can also chop the chillies if you want your soup more spicy, otherwise leave whole.
Throw all the herbs into a pot of boiling 100ml water. Boil for 2-3 minutes for the herbs to release flavours. Remove the hard central stalk of kaffir lime leaves and add the leaves to the pot.
Once the soup is boiling, add fresh peeled prawns. Let the soup heat back up for 1 minute, then add sliced eryngii mushrooms. Once the soup is boiling again, remove from heat.
Flavour the soup with lime juice and Thai fish sauce to taste. To balance the flavour, add the equal amount of lime juice and fish sauce e.g., 1 tbsp of each or 1.5 tbsp of each.