Marinate the fish with salt, red chilli powder and garam masala for 30 mins. Keep aside. After this time, deep fry the fish.
For the gravy, grind 2 onions, tomatoes, garlic and make the paste with 2 tbsp mustard seeds, 1 and tbsp cumin seeds. Soak it prior for atleast 2 hrs before cooking, if there is shortage of time soak in warm water for 30 mins.
Take a kadhai, put in 2 tbsp mustard oil, after it's warm add the paste and keep stirring, then add potato. Once you see the layer of oil on top of the paste, then add mustard seeds and cumin seeds which were soaked after the oil separates again.
Add 2 glasses of water, put in the potatoes, then cover and cook for 15 mins. Remove the lid add the fried fishes and coriander leaves to cook for 2 minutes.
Enjoy with steam rice.