I was challenged this evening with two ingredients: a stick of lemongrass and a box of pollack (white fish). So I've never tried it before, but it turns out that a tom-yum style soup is actually quite easy. I didn't have all the posh ingredients that I should have done (e.g. fish sauce) but we do have a bag of lime leaves in the cupboard, and the lime leaves give a really nice flavour.
It's a one-pot meal if you stick some noodles in the pot as well - here's what I did (serves 2):
(By the way - if you're not into hot flavours, reduce the amounts of the chilli and the ginger.)
Finely chop the ginger. Bash the lemongrass up and down with the butt of your chopping-knife. This helps release the flavour.
Put the water in a pan and bring it to the boil. Put in the lemongrass, lime leaves, chilli flakes, ginger and sugar, then turn the heat right down and let the mixture sit warming gently for 10 minutes to infuse.
Meanwhile, wash the button mushrooms, and chop the pollack into big bite-size pieces. Add the pollack to the pan, and turn up the heat. (When you add the fish it cools everything down, so you need to bring it back up a bit.) Add the mushrooms too, and when the water's bubbling again, turn the heat back down to a low setting.
Put the noodles into the broth (push them to the bottom so they'll soak nicely). Now let everything cook gently for about 6 or 7 minutes.