Wash the fish pieces. Pat dry with paper towel, marinate with turmeric, salt and red chilly powder for 10-12 minutes.Take a broad frying pan and heat mustard oil till smoking point, reduce the heat and gently lower the fish into the oil. Fry fish pieces on both the sides till it is light brown. Now very carefully without breaking the fish pieces place them on paper to soak extra oil and keep aside. Fry potatoes till they are light brown.
Now in another pan heat some oil thoroughly and add onion seeds and 2 green chilies and saute for 1 minute. Add Ginger paste and fry till it turns light brown. Add turmeric powder, cumin and chilly powder, salt n sugar.Fry for a minute and do no let the masala burn. Add tomato let it be cooked. Now add 1 1/2 cup of water, fried potato and adjust the salt. cover and let this cook for 5-6 minutes.
Gently slide in the fried fish pieces to the gravy, rest of the green chilies and cover it again for 2-3 minutes. Remove from flame. Add coriander leaves and cover the lid again. Serve this simple fish curry with piping hot rice.