Soak the tomatoes in boiling water for a few minutes. Rinse under cold water. Remove skin. Blend and set aside. Wash the fish and make a few tiny slits in each piece and set aside
Heat oil in a large flat pot on medium heat. Add the mustard seeds and when it pops add the onion and garlic. Turn the heat to a low setting. Sauté until the onion is golden brown
Add the chilli and curry leaf and fry for a minute
Add the chilli powder, cumin, masala and turmeric and fry the spices for a minute. Add a few drops of water to prevent the spices from burning
Add the tomatoes and season with salt. Bring to the boil
Lower the heat, cover and simmer until the oil surfaces to the top, about 15-20 minutes
Mix the tamarind in 1/2 cup hot water and put it through a strainer. Add it to the curry. Allow it to cook for 5 minutes
Add the fish but do not mix. You can baste the fish with the gravy. Cover and allow the fish to cook, about 5 to 8 minutes or until the fish is cooked. The time will depend on the thickness of the fish
Turn off the heat and garnish with coriander