Crab, tomato and lemon grass broth soups

6 lemon grass stalks
12 large ripe tomatoes
2 small fresh red chilli peppers
4 red shallots or 1/2 red onion
5 cm (2 in) piece fresh ginger
4 garlic cloves
12 cups (3 litres/4-3/4 imp. pints) fish stock
1-1/2 cups fresh coriander (cilantro) leaves
20 fresh lime leaves
Pinch of saffron threads
4 whole white peppercorns
4 whole coriander seeds
2 tablespoons fish sauce or to taste
2 cups (500 mL/16 fl oz) Clear Tomato Extract
Soup Garnish:
2 medium tomatoes extra diced
1 large fresh red chilli pepper finely sliced
4 fresh lime leaves finely shredded
60 g (2 oz) per person crab meat
1/4 cup fresh coriander (cilantro) leaves
1/4 cup fresh mint leaves

1. Peel ff the outer layers of the lemon grass, leaving the tender centre part. Roughly chop this. Chop the tomatoes, chilli peppers and shallots or onion. Peel the ginger and slice it. Peel the garlic and crush the cloves.
2. Bring the fish stock to the boil in a large saucepan over a high heat. Add all the soup ingredients except the fish sauce and the tomato extract. Reduce the heat to low and simmer for 45 minutes. Strain the soup through a fine sieve, discard the solids. Transfer the soup back to the saucepan, reheat and add the fish sauce and the tomato extract.
3. To serve, divide the tomatoes, chilli pepper, lime leaves and crab meat between the soup bowls. Ladle the hot soup over the garnish and serve topped with the coriander and mint leaves.