Heat coconut milk in pot over medium heat till just under boiling.
Add lemongrass, galangal (Tip: as you're going to have to remove these before serving, you can tie them up in cheesecloth, then just remove the cheesecloth when soup is done)
Add onion
Let simmer for about 3 minutes. Stir often to ensure that the coconut milk doesn't burn at the bottom of the pot.
Add chicken. Cook for 3 minutes.
Add mushroom and kafir lime leaves. Let simmer for another 2 - 3 minutes, stirring occasionally.