Put stock/broth together with lemongrass, galangal, kaffir lime leaf and salt into the saucepan, bring to a full boil over medium heat.
Add mushrooms and bring to a full boil again
Add prawns (shrimp) and leave to cook until shrimp are pink.
Do not stir.
Skim off fat/foam from the shrimp.
Next, add milk, stir a little then turn off the heat.
Add the flavors of the soup with the Thai chili paste, fish sauce, lime juice, sugar and crushed chilies.
Stir to blend well.
Note: Do not put lime juice in the soup over heat.
The lime will turn bitter.
Garnish with coriander leaves and serve.