Roughly crush the lemongrass and kaffir lime leaves, to allow the juices to release. Boil the galangal, kaffir lime leaves, lemongrass and mushrooms with water in a deep pan. Add tomato and spring onions. Cook for 1 minute and turn off the heat. Add lemon juice, chilli flakes, red chilli rounds, salt and soy sauce.
Garnish with a green chilli or coriander leaves before serving.