Masala (Grind these ingredients together):
Add the cooking oil, season with mustard seeds, ginger and garlic paste, add the onion slices and then the green chillies into a kadai (deep pan). Sauté till onions turn transparent. Then add the ground masala and sauté till an aroma arises. Now add the chopped tomatoes if so desired and toss for a minute. Add 1-1 ½ cup of water to the masala over medium heat, mix well and let it boil. Lower the flame. Add the thick tamarind pulp and let boil again. Then slowly add the fish. Toss the fish till it mixes with the gravy without breaking up. Cover and cook 10 minutes.
Taste for salt and sourness if desired.
Open and cook for another 2 minutes and put off the flame. Don!!!t remove the lid for 5-7 minutes.
Pour on to serving dish and garnish with chopped coriander leaves and a split green chilly.