Place the lemongrass onto a chopping board and smash it with the back of a heavy knife (this helps to release the flavors). Then cut the lemongrass into 4-5 pieces. Slit the Thai chilies lengthwise. Slice the galangal into thick slices. Peel and dice the shallot. Tear the kaffir lime leaves from the middle to help release the aroma.
In a pot, add the vegetable stock and all the aromats that you prepared earlier. Put it on medium heat and bring to a boil. Reduce heat and let the stock simmer for 10 minutes.
Add salt, sugar, mushrooms (separate the stalks) and coconut milk. Bring it to a simmer and let cook for 5-7 minutes. Do not boil rigorously after adding coconut milk, otherwise the fat will accumulate on the surface.
Turn off heat. Add lime juice, coriander and Thai basil (if using). Serve hot.
