Hot & sour mushroom soup

1.2 litres (2 pints) fish stock

1 lemon grass stalk, lightly crushed

3 fresh kaffir lime leaves or 3 pieces of lime rind

2 Thai red chillies, halved and deseeded

2 tablespoons lime juice

2 tablespoons Thai fish sauce

50 g (2 oz) can bamboo shoots

125 g (4 oz) oyster mushrooms

2 spring onions, finely sliced

½ red chilli, sliced, to garnish