Remove the scales, head, cut the fins and clean the mackerel. Wash and cut the mackerel into 3 to 4 pieces. Marinate by applying turmeric powder, red chili powder and salt. Keep the marinated mackerels aside for 20 minutes.
Soak tamarind in 5 tablespoon warm water and keep aside for 20 minutes. Mash and extract tamarind pulp. You may also use store bought tamarind paste.
Take a small bowl and mix, tamarind pulp, red chili powder and turmeric powder. Keep it aside.
Add oil to a saucepan. Once the oil gets heated, add mustard seeds. When it splutters, add crushed asafoetida/powdered asafoetida, fenugreek seeds and roast for 5 minutes. Add chopped garlic and fry till aroma of garlic gets filed in air. Switch off the gas and let this cool.
In the blender or mixer, add all the ingredients from the pan that is – garlic, asafoetida, some mustard and fenugreek along with little oil (from the pan) and grind/blend it to a coarse paste without adding water.
Retain some mustard seeds in the pan along with some oil.
Now, re-heat the pan in which some mustard seeds and oil is already present.
To this add the ground blended paste.
Next step is to add the tamarind and red chili paste.
Add the marinated mackerel pieces in the pan.
Add 1/ 4 cup water if needed. Do not add too much water. Pickle should be thick.