Clean oyster mushrooms and, depending on size, leave whole or cut into bite-sized pieces. Wash the tomatoes and remove the stalk. Cut the tomatoes into slices. Wash the chicken fillet, dab dry and cut into approx. 1 cm thick strips. Wash the chilli, cut lengthwise and remove the seeds. Cut the chillies into fine rings. Wash lemon grass and kaffir leaves, shake dry.
Cut lemon grass into pieces of about 3 cm. Wash galangal, peel and cut into thin slices. Put coconut milk, lemongrass, kaffir leaves and galangal in a large pot, bring to the boil and simmer over medium heat without lid for about 2 minutes. Add chicken stock and bring to the boil. Add chicken fillets, oyster mushrooms, tomatoes and chili and let simmer for about 5 minutes. Wash Asian chives, shake dry and cut into fine rolls. Season finished chicken soup with fish sauce and lime juice