In a medium-sized pot, bring the chicken broth to a simmer over medium heat. Add the galangal (or ginger), lemongrass, and kaffir lime leaves. Simmer for 5-7 minutes to infuse the broth with the aromatic flavors.
Stir in the coconut milk and bring the mixture to a gentle simmer. Do not let it boil, as coconut milk can curdle at high heat.
Add the thinly sliced chicken and mushrooms to the soup. Cook for about 7-8 minutes, or until the chicken is fully cooked and tender. The chicken will cook quickly due to the thin slices.
Stir in the fish sauce (or soy sauce), lime juice, and sugar (if using). Taste and adjust the seasoning as needed, adding more lime juice for sourness or fish sauce for saltiness.
Add the sliced Thai chilies to the soup. Adjust the quantity based on your desired level of spiciness.
