Mash the prepared green and red chillies in a mortar and grate gently. Mix with the fish sauce and lime juice, set aside.
Bring the water to the boil with salt, sugar, lime leaves, galangal and lemongrass. Reduce the heat and add the whole chicken breast fillets. Depending on the thickness of the fillets, simmer gently for 5-10 minutes until they are cooked through. Remove the chicken fillets and let them rest.
Quarter or eighth of the mushrooms depending on their size, add to the soup and let simmer for approx. 4 minutes.
In the meantime, divide the chicken breast fillets lengthways and cut across into slices.
Put the fish and lime sauce with the chillies in a serving bowl, spread strips of meat over it and fill up with the broth.
If necessary, season with a little lime juice and fish sauce. The soup should taste hot, sour and salty.
Pour the soup into portion bowls at the table and sprinkle with the coriander leaves. You can also serve the meat, tom yam and the sauce separately at the table. So everyone can fill the soup to their own taste and season with the sauce.
The lime leaves, lemongrass stalks and galangal slices are served, but not eaten.
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