Peel and roughly dice the onion. Wash the chicken breast. Put it in a pot with diced onion, bay leaf, approx. 1 tsp salt and peppercorns. Add approx. 1 l cold water, so that the meat is covered.
Bring everything to the boil and simmer at low heat for about 45 minutes. From time to time skim off the resulting foam.
Cut the chilli lengthwise, remove the seeds, wash and cut into fine rings. Remove the outer leaves from the lemon grass. Wash the galangal. Cut both into large pieces. Wash lime leaves and tomatoes.
Clean mushrooms and wash them briefly if necessary. Wash the pumpkin, cut into slices, remove seeds and dice with the skin. Clean and wash spring onions and cut into thin rings.
Lift out the chicken breast and let it cool down. Pour broth through a fine sieve and collect in a pot. Add coconut milk and all prepared ingredients except tomatoes and spring onions. Bring to the boil again and simmer covered for about 10 minutes.
In the meantime, remove the meat from the skin and bones and cut or pluck it into bite-sized pieces. Wash the coriander, shake dry, pluck off the leaves. Add spring onions, tomatoes, chicken and coriander to the soup 2-3 minutes before the end of cooking time.
Season soup with fish sauce. Add rice.