Add the shrimp stock, lemongrass, kaffir lime leaves, galangal and chilies to the pot. Bring to a boil and let it boil for 3-4 minutes until you can smell the fragrance of the herbs from the pot.
When the soup is done simmering, add the oyster mushrooms, and bring the soup back to a boil.
Once the soup comes back to a boil, add the shrimp and when the soup just starts to bubble again, turn off the heat. Let the residual heat of the soup cook the shrimp completely, another minute or so.
Add the lime juice, fish sauce, chili paste (if using) and sugar to your soup and stir. Taste and adjust seasoning to your liking. If you’re not using the chili paste, you may find you need to add a bit more fish sauce/sugar. But taste it first!
Garnish with cilantro leaves and serve with rice