In a pot, bring the chicken stock to a boil. Add the kosher salt and chicken pieces, then reduce the heat to a gentle simmer. Cook for 15-20 minutes until the chicken becomes tender and easily pierced with a fork.
Pour in the coconut milk, then add the lemongrass, galangal slices, makrut lime leaves, Thai chilies, half of the fish sauce, and the palm sugar. Allow the mixture to simmer for 5 minutes, letting the flavors blend.
Stir in the oyster mushrooms, bringing the soup back to a simmer. Cook for an additional 1-2 minutes, just until the mushrooms are tender.
Taste the soup and adjust seasoning with more fish sauce if necessary. Remove the pot from the heat, add 2 tablespoons of lime juice, and taste again, adjusting the lime juice to your preference. Garnish with chopped green onion and/or cilantro.
Serve with jasmine rice as a part of a Thai meal, or enjoy by adding rice directly into the soup for a complete meal. (Note: The herbs are used for flavor infusion and are not meant to be eaten. Either remove them before serving or inform your guests accordingly.)