Mix 1/2 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 pop add the cardamom pods, green chillies and curry leaves. Add the onion, ginger and garlic paste and fry for a minute or two.
Add the remaining ground turmeric, the water, sugar and salt. Mix well, then simmer for a minute. Reduce the heat to low, add the fish steaks and cook for a further minute.
Add the coconut milk then cover and simmer gently for 4-5 minutes or until the fish is moist and cooked through. Swirl the pan gently to mix a couple of times but make sure the fish pieces dont break up.
Add the tamarind, black pepper and fresh coriander and stir gently, then simmer for a minute. Serve with some basmati rice and a cucumber salad.