Prepare the Base: Combine chicken stock with sliced galangal and kaffir lime leaves in a pot. Bring this mixture to a gentle boil.
Add Proteins and Vegetables: Introduce the sliced chicken to the simmering liquid. Cook until it is no longer pink. Then, add the mushrooms and cook for another few minutes.
Introduce the Richness: Pour in the full-fat coconut milk. Stir gently to combine all ingredients. Do not allow it to boil vigorously after adding the rich liquid.
Season and Balance: Stir in the brown sugar to balance the tastes. Add a dash of fish sauce for depth. Taste and adjust as needed.
Finish and Serve: Remove the galangal and kaffir lime leaves before serving. Garnish with fresh cilantro. Serve hot and enjoy your homemade soup.