Smash the lemongrass, galangal, and shallots to release their natural oils and fragrance.
In a medium pot, combine coconut milk, water, smashed aromatics, and torn kaffir lime leaves. Bring to a gentle simmer over medium heat.
Once the soup begins to simmer, add sliced chicken and mushrooms. Cook until chicken is fully done (about 5–7 minutes).
Stir in tamarind juice, sugar, and salt. Taste and adjust seasoning as needed. Optionally, add sliced Thai chilies for extra heat.