A fragrant Thai soup balancing sour, salty, spicy, and sweet, built on a broth of lemongrass, galangal, and kaffir lime, finished with prawns cooked gently in residual heat.
Bring the water to the boil in a saucepan. Meanwhile, chop the mushrooms and bring to the boil.
Chop the tomatoes and slice the galangal. Chop the lemongrass a little smaller to fit into the pot. With the blade of a knife, smash the garlic and use with the peel on.
Put the rest of the ingredients, except the prawns, into the pan and bring to a good boil for five minutes. Check the taste.
Pour in the frozen prawns and put the Tom Yum soup on the table under the lid to rest for a while. The prawns will cook quickly and cool the soup down to an edible consistency.
Garnish the finished Tom Yum with torn coriander.
Notes
Bruise or smash the lemongrass and galangal before adding them to release more of their fragrance into the broth.
Taste as the broth simmers and adjust lime, fish sauce, or sugar to balance sour, salty, and sweet.
Take the pot off the heat before adding the prawns — the residual heat cooks them gently without turning them rubbery.
Add coconut cream for the creamy Nam Khon style, or leave it out for the clear Goong version.
Tear the coriander by hand rather than chopping it for a more rustic garnish.