Peel and devein prawns keeping the shells and heads aside for the stock. Slice ¾ of the way through to butterfly them so they curl when cooked. Save the prawn shells for your stock.

Bash and chop the lemongrass into 2 pieces. Chop mushrooms into quarters and slice shallots, galangal and chillies.

To make the prawn stock, place all the shells and heads in a pan with a little oil and fry off the shells until they turn pink. Push down on the heads to release their oils. Add the water and leave to simmer for 15–20 minutes.

Using a slotted spoon, remove the shells from the stock you have now created. Add the lemongrass, lime leaves, galangal and chilli to the stock and simmer for 5 minutes.

Add the evaporated milk and let it come to boil. Add the mushrooms to the soup and cook for 1–2 minutes.

Add the chilli paste and fish sauce, stir through and then add the prepared prawns and let them poach in the soup for 30 seconds. Make sure the soup isn’t boiling at this point so not to overcook them.

Turn the heat off and allow to cool for a few minutes before adding the lime juice. This is so that the lime juice doesn’t curdle the evaporated milk. Add the lime juice and stir through.

Serve the soup in bowls with fresh coriander and extra lime if necessary.