Take half of the Zanzibar fish masala and blend in a processor with the chopped onions, vinegar, lime juice and green chillies until a paste forms. Cover the prawns with the paste and leave to marinade for 30 minutes.
Heat 2 tbsps sunflower oil in a large frying pan. Add the prawns with the marinade and fry until they are cooked through, then remove from the wok.
Add another 2 tbsps sunflower oil and fry the sliced red onions until they start to brown. Add the remaining Zanzibar Fish Masala and fry for 30 seconds, stirring often.
Add the tomatoes and fry for 3 minutes then add 150 ml of water and salt to taste. Simmer until the oil returns and the sauce reduces.
Return the prawns to the wok and simmer for a further 3 minutes.
Garnish with the coriander leaves and serve with rice.