Sour Prawn & Mussel Soup (Tai)

INGREDIENTS (serves 8)
1.5L (6 cups) cold water
2 lemon-grass stems, pale section only, thinly sliced diagonally
8cm-piece fresh galangal, peeled, thinly sliced
5 purple eschalots, peeled, thinly sliced (sustitute with 1 red onion)
6 fresh kaffir lime leaves, centre vein removed, torn
500g medium green prawns, peeled leaving tails intact, deveined
500g black mussels, scrubbed, debearded
80ml (1/3 cup) fish sauce
125ml (1/2 cup) fresh lime juice
2 fresh red birdseye chillies, thinly sliced
1 bunch fresh coriander, leaves picked, coarsely chopped

METHODS
1. Place water in a large saucepan and bring to the boil over high heat.
2. Add lemon grass, galangal, eschalot and lime leaves and cook, uncovered, for 5 minutes.
3. Add prawns, mussels and fish sauce and cook, covered, for a further 2 minutes or until mussels open. Remove from heat. Discard unopened mussels.
4. Add the lime juice, chili and coriander and stir to combine. Ladle soup among serving bowls and serve immediately.
--From Australian Good Taste