1) In a very little oil , saute chilli , whole coriander and jeera until they are crisp. Soak tamarind in water and mash to a pulp, remove the seeds or dilute tamarind paste with 1/4 cup water.
Mash the roasted masala along with tamarind into a smooth paste. In a pan, heat the rest of the oil; add the mustard seeds and curry leaves , and cook until they pop.
Add the chopped onion and tomato and saute well ; add taute ( yellow cucumber ) , turmeric powder and red chilli powder. Add water and cover the pan; simmer until the taute is cooked. Add ground masala and cook until the oil separates. Stir in jaggery and salt to taste.
Fry masala ingredients from 2 to 7 and then grind with ingredients from 2 to 11 to a fine paste with just enough water to grind. You can add 1/2 teaspoon hing powder if desired while grinding. Remove the masala and keep aside.
Take chopped green chillies , ginger , garlic and mix in coconut oil. Then add this mixture to the masala.
Clean the mackerels and make 4 to 5 slits in each. Coat the mackerels with the masala and keep them in a flat , greased baking dish and bake at 200 degrees C for 20 to 25 minutes until dry or you can cook in a greased thick kadai / earthen pot on a low heat .
Decorate with onion rings and lemon wedges.