Pour Coconut Milk (3 cup) a pot. Add Galangal (6 slice) and Lemongrass (3 stalk). Set the pot on the stove over medium heat. Hint: Tie the galangal slices in cheesecloth so they can easily be removed.
When the soup begins to simmer, add Onion (1).
Allow the soup to simmer for 1 minute while stirring often and scraping the bottom of the pan to ensure the coconut milk does not burn.
Add Chicken Thigh (1 cup), then let the soup simmer for 2 minutes. If the soup gets too thick, add water (1/2 cup at a time) until it reaches your desired consistency.
Add Mushroom (1 cup), Red Bird's Eye Chili Pepper (3), Cherry Tomato (1/2 cup), Kaffir Lime Leaf (3), Lemon Juice (2 tablespoon), Fish Sauce (3 tablespoon), Salt (1 teaspoon), and Granulated Sugar (2 tablespoon).
Allow the soup to simmer on low heat for about 5 more minutes while stirring often to prevent the coconut milk from burning.
Transfer to serving bowl(s). Garnish with Fresh Cilantro (1 bunch) just before serving.