Tom Yum Kung

Method

1. Peel 4 large prawns, keeping the heads and shells and reserving them. Slit each prawn down the back and lift out the dark vein, then set the peeled prawns aside on a plate.
2. Put the reserved shells (not the heads) in a saucepan with 300 ml water, bring to the boil over high heat and simmer gently for 10 minutes, skimming any foam. Strain, discard the shells and return the stock to the rinsed pan; you want about 300 ml.
3. Bruise 1 stalk lemongrass with the flat of a knife and cut it into 4 cm lengths on the diagonal. Add it to the stock with 5 slices galangal slices, 3 kaffir lime leaves torn in half, 3 bird's eye chillies lightly crushed, and 2 small shallots halved and crushed. Bring to a bare simmer over medium heat and infuse for 5 minutes — do not boil hard or the broth turns cloudy.