1 fresh long red chilli, seeds removed, sliced thinly
If making your own chicken stock, rinse the chicken well under running water. Place into a large stockpot and add enough cold water to just cover the chicken. Bring to the boil, uncovered, skimming off any scum with a spoon.
Add the ginger and leeks, reduce the heat to a simmer and cook for 1½-3 hours, until the broth has taken on the flavours of the ingredients. Strain into a clean bowl and leave to cool.
For the soup, place a large saucepan on the heat and add the oil and one of the chopped tomatoes. Cook for three minutes until just softened.
Add the fish sauce, lemon juice, star fruit and pineapple and stir well. Add the freshly ground black pepper, tamarind, and sugar, to taste.
Add the chicken stock, spring onions and remaining chopped tomato and bring to a rolling boil. Boil for two minutes, then add the prawns and cook for a further minute, or until the prawns have turned pink and are cooked through.
Remove from the heat and stir in the coriander leaves and chilli slices. Serve immediately.