Marinate the fish in 1/2 a tsp of turmeric powder and little bit of salt and keep aside.
In a frying pan, take 2 tbsp of mustard oil. When the oil is hot, lightly fry the fishes. Frying koi mach is a bit tricky. Koi mach is always bought live and sometimes while frying, the fish (even though dead), jumps out of the pan. So, while frying, make sure to keep the lid on.
Make a paste of red chili powder, coriander powder, cumin powder, 1 tsp of turmeric powder, mustard paste, tomato paste, ginger paste, 2 tbsp of mustard oil and some salt.
Tomato paste is something which can be used as per your taste. I use the tomato paste since it gives an additional red colour and makes the gravy quite rich. My grandmother never used tomato paste.
Once the fish has been fried, use a slotted spoon to take them out of the frying pan and keep them aside.
In the same frying pan, add 3 more tbsp of mustard oil. When it becomes hot again, add the masala paste. Initially keep the heat on high and fry the masala for a couple of minutes. Then lower the heat and cook the masalas till the raw smell goes off and oil releases from the sides.
Add the fish and coat them with the masalas. Keep the heat low so that nothing burns.
Add a cup of water and then increase the heat and let it simmer.
Add some split green chillies. And finally drizzle some more mustard oil. Be generous with the mustard oil. Check the seasoning and add more salt if required.
The recipe is called tel koi (tel meaning oil) , hence there is more oil than water.
Serve it with some steaming rice.