Wash the fish properly and keep it aside.

Now heat the oil in a kadhai, put the cumin seeds, green chilli and diced tomatoes into it.

Meanwhile, add the turmeric powder to the fish and mix it properly.

Now take a bowl and make a semi thick paste of mustard powder and water.

Pour this paste in the kadhai, when the tomatoes are sauted.

Fry for 3-4 minutes and then add the fish.

Now, add the curd to it, along with sugar and mix it slowly.

Keep frying untill you see oil coming out from the gravy (7-8 minutes). Now add salt to taste and pour some water into it.

Simmer for 3-4 minutes. Serve hot..!!

NOTE: Illish is a very delicate fish, handle it with care while frying or it will break very easily. Also the sugar should be in a very less amount because it's the taste of salt that prevails.