Clean the fish and rinse it couple of times till water comes clear.
Heat 2 cups of water along with 1/2 tsp turmeric powder, salt, and tamarind water. When water is hot enough add fish and cook for 5 mins. Once done switch of the stove and allow it to cool.
In a grinder/blender take chopped tomato, ginger, garlic, green chilli, coriander leaves and grind it to smooth paste without adding water.
Once fish is cooled down, separate the flesh from thorns and keep it aside.
Heat 2 tbsp oil in pan in medium heat. Add chopped onions and fry till it turns translucent.
Now add masala paste and fry till all liquid dries off.
To this add turmeric powder, garam masala powder, asafoetida, salt and mix well.
Add mackerel flesh and mix it. Cook it for another 2-3 mins. Put off the stove and allow mixture to cool.
Once the mixture is warm enough to touch, make round cutlets.
Heat tawa/griddle, add oil. Once oil is hot enough fry cutlets till both sides turn golden.