Cut the heads off the prawns and peel the tails leaving the end of the tail intact. Place shells and heads into a saucepan with the torn lime leaves. Place the chillies, galangal, garlic and lemongrass into a mortar and pestle (see note) and use the pestle to bruise the ingredients - you are not making a paste, just giving them a bit of a whack to release the flavours.

Add the chillies, galangal, garlic and lemongrass to the saucepan, cover with 750ml of water and add the chicken or vegetable stock. Bring to a simmer, then cover with a lid left slightly ajar. Turn the heat right down and simmer gently for 10 minutes.

After 10 minutes, strain the stock, discarding the solids, and return the stock to the saucepan. Stir through the chilli paste, then add the mushrooms, onion and tomato and simmer for 3 minutes. Add the prawns and cook a further 3 minutes, then stir in the fish sauce and sugar.

Add the lime juice and adjust seasoning to taste. With all Thai food, there is a balance of spicy, sweet and sour, so it’s important to taste the soup and alter the recipe by adding more chilli, lime juice, fish sauce or sugar depending on what you like.

Divide between serving bowls and garnish with coriander, chilli and finely sliced kaffir lime leaves. Serve with rice or noodles.