Peel the prawns, reserving the shells. Devein the prawns and set aside. Rinse the shells under cold water, then put them in a large pan with the stock or water. Bring to a boil.

Bruise the lemongrass stalks and add them to the stock with half the lime leaves. Simmer gently for 5 to 6 minutes, until the stock is fragrant. Strain the stock, return it to the clean pan and reheat. Add the drained mushrooms and the prawns, then cook until the prawns turn pink.

Stir the fish sauce, lime juice, spring onion, cilantro, chilies, and the remaining lime leaves into the soup. Taste and adjust the seasoning, if necessary. The soup should be sour, salty, spicy and hot.

Serves 4 to 6