Mix ½ tsp of the ground turmeric, with the salt and lime juice and rub into the fish steaks, then set aside. Add the onion, ginger and garlic to a blender and blend to a smooth fine paste with a splash of water. Set aside.
Heat the oil in a heavy-based, non-stick saucepan over a medium heat. Add the mustard seeds and as they begin to sputter, work quickly to add the cardamom pods, green chillies and curry leaves.
Add the onion, ginger and garlic paste and fry for 1–2 minutesm
Add the remaining ground turmeric, along with the water, sugar and salt. Mix well, then simmer for 1 minute.
Reduce the heat to low, then add the fish steaks and let them cook for a further minute. Add the coconut milk, cover and simmer gently for 4–5 minutes until the fish is moist and cooked through.
Add the prawns. Give the pan a gentle swirl a couple of times to make sure everything is mixed but the fish pieces don’t break up.
Add the tamarind, black pepper and fresh coriander and stir well, simmering for 1 minute. Serve with plain basmati rice and a cucumber salad.