Pour the chicken bone broth into a pot and bring to the boil.
Drop the galangal slices, lemongrass, kaffir lime leaves and chilies into the broth. Lower the heat. Cover and simmer for 15 minutes.
Add the chicken thigh fillet slices, oyster mushrooms and tomatoes to the broth.
If the broth is unseasoned or underseasoned, add fish sauce at this point.
Cover the pot once more and simmer for another 15 minutes.
Taste the broth. Add more fish sauce, if needed.
Drizzle in the lime juice, a tablespoonful at a time, tasting and adding as you go along until a good balance of salty and sour is achieved.
Ladle the tom yum gai into bowls (see notes after the recipe), sprinkle with cilantro and serve.