De-scale, clean and cut the mackerels into 3 pieces. Discard the head.
Marinate them by applying red chili, turmeric power and salt and keep it in fridge (not freezer) for 30 minutes.
In a kadai or wok, heat 1 tablespoon oil. Stir fry rice till it turns brownish, remove and keep it in separate plate.
Then in the same oil, stir fry urad dal till it turns brownish, remove and keep it aside.
To the same oil add fenugreek and peppercorns together. Stir fry till fenugreek changes color, remove and keep it aside and switch off the gas. Let this cool.
Using a mortar-pestle coarse or powder rice and black gram and keep it aside.
Coarse fenugreek seeds and peppercorns in the same mortar-pestle and keep it aside.
Chop half the onion length wise. Switch on the gas and in the same oil stir fry onion till it turns pinkish. If needed add 1 teaspoon oil. To this, add grated coconut and fry on low flame till it turns brownish and switch of the gas. Let this cool down.
After cooling add the fried onion and coconut mixture from the kadai to a mixer jar along with tamarind, turmeric powder and dry red chilies and grind it into a thick gravy by adding 3/4th cup. Ensure that all the ingredients are ground well.
Heat a kadai and add oil. Cut the remaining half of the onion into small pieces and fry it. Pour the gravy from the mixer to the kadai and mix well. Add 1 cup water.
Add marinated mackerels and stir. Now add the coursed black gram, rice, fenugreek and peppercorn to the gravy. Simmer for 10 minutes.