Lightly marinate chicken with salt. Arrange on a heatproof dish and steam over high heat for 15 minutes. Remove and set aside.
Combine coconut milk, lemongrass, galangal, bird's eye chillies, kaffir lime leaves, shallots, coriander roots, and black and white peppercorns in a pot. Bring to a boil over medium low heat for 10-15 minutes.
Add pre-steamed chicken and the stock to the soup stock. Add all mushrooms, thick coconut milk and seasoning. Stir well and cook for 4-5 minutes. Dish out soup into individual bowls and serve hot.