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.