Heat a heavy-bottomed pan on a medium flame and roast the dry red chillies, mustard and cumin seeds. Cool and grind along with tomatoes.
Heat the oil in a wide, heavy-bottomed pan and add the curry leaves. When they stop spluttering, add the onion paste and fry till light brown.
Add the ginger and fry for a minute.
Add the tomato and spice paste and fry till the oil begins to separate from the masala.
Add 2 cups of hot water and the tamarind pulp to this masala and stir to mix well.
Bring the gravy to a boil and then simmer till u get desired consistency. N likes thick gravy in fish dishes, so I boiled it for about 7 minutes or so.
Gently add the fish to this gravy and cook till done.
Garnish with chopped coriander and serve hot with plain boiled rice.