Rinse the prawns in cold water and drain well. Heat the oil in a large saucepan or wok, add the onion and fry for 5 minutes until just beginning to brown.
Stir in the garlic, ginger and tamarind paste, then mix in the lime juice and stock.
Bring the stock to the boil, add the prawns and cook, stirring, for 5 minutes until the prawns are bright pink. Spoon into bowls and serve garnished with the torn coriander leaves and lime wedges.