Sweet and sour fish soup

Ingredients:
60 g (2¼ oz/¼ cup) tamarind pulp
2 lemongrass stems, trimmed
2½ tablespoons chopped palm sugar (jaggery)
60 ml (2 fl oz/¼ cup) fish sauce
1.5 litres (52 fl oz/6 cups) chicken stock or water
½ small ripe sweet pineapple, trimmed, cored and cut into small chunks
750 g (1 lb 10 oz) firm white fish fillets (such as ling, blue eye or snapper), trimmed and cut into 2 cm (¾ inch) chunks
4 small tomatoes (about 500 g/1 lb 2 oz in total), cut into 2 cm (¾ inch) chunks or thin wedges
300 g (10½ oz) sugarsnap peas, trimmed
2 spring onions (scallions), thinly sliced
1–2 small red chillies, thinly sliced
3 kaffir lime leaves, very thinly sliced (optional)
herbs such as mint, coriander (cilantro), Thai basil and Vietnamese mint, to serve
trimmed bean sprouts, to serve

Method
1) Put the tamarind pulp in a small heatproof bowl and pour 375 ml (13 fl oz/1½ cups) boiling water over. Stir with a fork to loosen the tamarind and leave to soak for 15 minutes, or until the tamarind has softened. Push the mixture through a sieve into a very large saucepan or stockpot, discarding the solids.
2) Cut the lemongrass stems in half lengthways, then bruise them with the back of a large knife. Tie the lemongrass pieces into knots, or fold them into a loose bundle and secure with kitchen string. Add to the pan with the palm sugar, fish sauce and stock. Slowly bring to the boil, then cover, reduce the heat to medium–low and simmer for 10 minutes.
3) Add the pineapple, fish, tomatoes and sugarsnap peas. Bring to a gentle simmer and cook, covered, for 2 minutes, or until the pineapple and vegetables begin to soften and the fish is cooked through. Season to taste with freshly ground black pepper, then stir in the spring onion, chilli and lime leaves, if using.
4) Divide the soup among warm bowls and serve immediately, with the herbs and bean sprouts passed separately.