Cook's Note: Galangal can be found at Asian markets; however, you may substitute an equal amount of fresh ginger if you cannot find it fresh.
Wrap the garlic, kaffir lime leaves, chiles, lemongrass and galangal in a piece of cheesecloth and tie with kitchen twine. Combine the chicken stock and the cheesecloth bundle in a small stockpot and bring to a boil over high heat. Lower the heat and simmer, uncovered, for 30 minutes.
Add the lime juice and fish sauce to the simmering stock and stir to combine. Sprinkle the chicken thighs with the kosher salt. Add the chicken to the pot along with the mushrooms and tomatoes, and continue to simmer until the chicken is just cooked through, for 4 to 6 minutes. Taste the soup and check for seasoning; it should be slightly salty and sour. Adjust as needed with additional fish sauce or lime juice.
To serve, ladle the soup into individual bowls and top with chopped cilantro. Serve with any or all of the suggested accompaniments.