Heat the oil over a medium heat in a heavy based saucepan then add the mustard seeds and fry for a minute until they start to pop.
Add the onion and half the curry leaves. Stir and fry for about 10 inutes until trhe onion is soft and golden brown.
Add the garlic and fry for a further 3 minutes, then add the ground spices and fry for about 30 seconds until fragrant.
Tip in the passata, tamarind liquid, chillies, vinegar and salt then add the prawns with about 3 tbsps water. Simmer for 4-5 minutes until cooked through. The curry can be left at this point for a few hours in a cool kitchen.
Stir in the remaining curry leaves and fresh coriander just before serving and serve with some plain boiled basmati rice.