Bring the water to the boil in a A saucepan. Lower the heat and add the fish and onion, with 2.5 ml (1/2 tsp) salt. Simmer gently until the fish is cooked through.

Remove the fish from the stock; set aside. Add the tamarind, fish sauce and sugar to the stock. Cook for 5 minutes, then strain the stock into a large jug or bowl. Carefully remove all of the bones from the fish, keeping the flesh in big pieces.

3.Heat the oil in a large frying pan. Add the garlic and lemon grass and fry for a few seconds. Stir in the tomatoes and bean paste. Cook gently for 5—7 minutes, until the tomatoes are soft. Add the stock, bring back to a simmer and adjust the seasoning.

Drain the vermicelli. Plunge it into a saucepan of boiling water for a few minutes, drain and divide among individual serving bowls. Add the beansprouts, fish, basil or mint, and sprinkle the ground peanuts on top. Top up each bowl with the hot soup.