Marinate the fish with the chilli powder, cumin, garlic and salt. Refrigerate for half an hour
Blend the onion, ginger/garlic paste, pinch of ground cinnamon, cumin and coriander into a fine paste. Set aside
Heat 1 tablespoon oil and add the curry leaf and mustard seeds. Once the mustard seeds start to burst add the ground paste and cook for 2 minutes
Add the chilli powder and turmeric and cook for a further minute
Pour in the coconut milk. season with salt and simmer on a low heat for 15 minutes until the sauce has reduced
Mix the tamarind in the tablespoon of warm water. Add it to the sauce and simmer for a further 5 minutes
Whilst the sauce is simmering sear the fish on both sides for about a minute each. Add it to the sauce and bring to a boil for 2 minutes. Season with salt. Turn off the heat. Be careful not to overcook the fish