Bring chicken stock to a boil. (if using chicken thighs add it now, if using breast it can be added later)
Add coconut milk, lemongrass, galangal, makrut lime leaves, chilies, 1 tbsp fish sauce, and coconut sugar. Simmer for 5-7 minutes to infuse. ( don't simmer for long as the flavors of the herbs vaporize)
Add the mushrooms, (if using chicken breast slices, add now) bring the heat back to a simmer, and cook for 5 minutes or until the chicken is cooked through.
Taste and add more fish sauce as needed. Take it off the heat and add 2 tablespoon of the lime juice, taste and add more as needed.
Garnish with chopped green onions and/or cilantro.
Serve over some steamed rice. And some lime wedges on the side.