Heat the oil in a saucepan. Add the mustard seeds, cook for 2 minutes over a medium heat. Stir in the onion, garlic, ginger and chillies and cook over a low heat until the onions are translucent.
Stir in the powdered spices then add the tomato and 125ml (4fl oz) water. Mix well and raise the heat to medium. Cover the pan and simmer for about 10 minutes until the tomato has broken down. Season with salt to taste. Add the prawns and cook for 5 minutes.
Stir the coconut milk into the sauce. Serve with naan bread or basmati rice.