Bring water to a boil in a pot, adding crushed lemongrass and sliced galangal.
Add mushrooms and Thai chili paste, simmering for 5 minutes.
Stir in prawns and cook for 3 minutes until pink and cooked through.
Remove from heat, then stir in fish sauce, fresh lime juice, and salt.
Garnish with chopped fresh coriander leaves and serve piping hot.