Wash the fish pieces nicely with water and drain it,for marination add 1 tbsp of salt and 1 tbsp of turmeric powder,mix it well and keep it aside.
Heat a pan adding with 1/2 cup of oil,once oil is hot add the marinated fish pieces and fry it nicely till it became golden in color from both the side and keep it aside.
In min while take the blender jar,add mustard seeds,cumin seeds,garlic cloves,and if u are taking dry red chili then add it with little water and blend it nicely to make a fine masala paste and keep it aside.
After frying all the fish pieces add little oil and fry the Potato pieces into it till it became golden fry and keep it aside.
In the same pan add little mustard oil,once oil is hot add the Panch phoron and let it crackle.
Once it start crackling then add the chopped onions and fry it till it became transparent.
Once onions fry then add chopped tomato,salt,turmeric powder,if you are using chili powder add that and nix it well and let it cook for some time.
Once onions became soft add the mustard paste into it and mix well then let it cook for 5 mints with low flame.
Now add the fried Potato pieces and mix it nicely for 2-3 mints,then add 1 glass of water and let it cook the potato.
After 5 mints check it ,if gravy start boiling add the fried fish pieces carefully and mix once slowly.
Then cover the lid and cook the fish gravy with medium flame for 7-10 mints.
After 7 mints check the gravy and potato if its well cooked then..You can adjust the gravy as per your requirement.
Then switch off the flame and sprinkle some chopped fresh coriander leaves into the gravy and keep it for 5 mints.
After 5 mints remove the lid and serve hot mustard gravy fish curry with plain white rice and enjoy you food.