Grind the coconut along with turmeric powder, Chilli powder and Coriander powder into a smooth paste.
Heat oil in a kadai and add mustard seeds. When they splutter add the fenugreek seeds.
Add the onions and saute. When they become brown here and there add the green chilli s and the tomatoes.
Then add the ground paste and mix well. Add the tamarind juice.Then add the canned mackeral and salt and mix well. Add 1/2 cup of water. Cut the mackeral into small pieces and add. Then only the masala will get into the fish.
Add curry leaves and close with a lid and allow it to cook for 15 -20 min.