In a pan add all ingredients except tamarind and fry till it turns dark brown. Now add tamarind to the mixture and take it in a blender and make into a smooth paste.
Transfer this to a kadai along with some water. Add salt to taste and bring it to boil. Cover and simmer till oil separates.
Now add in the fish pieces and mix well.Cover and simmer again for 8-10 mins.
Heat coconut oil in a pan and add chopped shallots and fry for 3 mins.