Bring water to the boil, add shallot, lemongrass, kaffir lime leaves and tamarind.
Use the back of the ladle smash the tamarind against the sauce pan to dissolve.
When boil, add mushroom. Season with fish sauce.
Leave it to the boil again.
Add river prawn and pounded chili and taste. Before turning off the heat, add the chili in oil and stir to dissolve.