Remove the head and shells of the prawns and wash. De-vein and wash them thoroughly under running water. Place the head and shells in a pan with four to five cups water.
Bruise lemon grass stems, two kaffir lime leaves and two green chillies in a mortar or tear them with hand and add to the pan. Heat the pan and bring the mixture to a boil, add salt, cover and simmer for fifteen minutes.
Take off the heat and strain. Slice the remaining chillies into fine roundels and reserve for garnish. Heat the strained stock and bring to a boil. Add the mushrooms and prawns and simmer for two to three minutes or until the prawns are pink and cooked.
Stir in the fish sauce, red curry paste and lemon juice to make the soup sharp and tangy. Adjust the seasoning.
Pour into a serving bowl and garnish with green or fresh red chilli roundels, whole kaffir lime leaves and coriander leaves. Serve piping hot immediately.