Remove the heads from the prawns, then shell the prawns, leaving the tail attached for presentation.
Start with 1.5 litres of water, add one tablespoon each of crab paste & prawn paste, one tablespoon of Tom Yum paste (optional: add more if you require more spice); add one tablespoon of kaffir lime leaves (or four whole leaves); add half a cup of finely sliced ginger and 2 tablespoons of finely sliced lemongrass;
Boil all for a half hour, then add the juice from 1 whole lemon, 4 tablespoons of brown sugar, the roots of 1 bunch of coriander and 12 prawn heads
Add 1 tablespoon of salt and 2 cloves of garlic, chopped finely. Boil for 10 minutes on high; remove prawn heads 5 minutes before serving, adding 1 thinly sliced carrot, 8 finely sliced button mushrooms, and the remainder of the coriander bunch, sliced thinly, and the 12 whole prawns, and add 1 teaspoon of salt to the pot.