Heat oil in a pan and add mustard and let splutter. Now add cumin, fenugreek, onion and green chilies and stir fry. To this add Corriander powder, Red chili powder, turmeric powder and coconut and stir well until coconut fries and a coconutty aroma starts coming out. Now add tomatoes, coriander leaves, curry leaves and salt. Stir till tomatoes are well cooked.
Add fish,water and tamarind paste to it and cook till done. Serve with steamed tapioca or Kerala parotta.