First, wash the Ilish fish.

Place the fish on a plate and add salt and turmeric powder. Coat the pieces of fish with the salt and turmeric powder and keep them aside.

While the fish is resting, soak the mustard and poppy seeds in a bowl.

After around half an hour, take a sil batta/mortar and pestle/blender and add the white mustard, black mustard, poppy seeds, two green chilies, and a little salt.

Make them into a fine paste and keep them aside.

Now whisk the sour curd/yogurt well until creamy.

After that, place the fish in a big steel tiffin box.

Add the paste of the spices and whipped sour curd.

Add salt to taste and combine everything nicely. Coat all the pieces of fish with the creamy paste and curd.

Arrange the fish inside the tiffin box and drizzle 2 tsp Mustard oil over the fish.

Slit the green chilis from the middle and place them on the pieces of Ilish fish.

Put the lid of the tiffin tox tightly and keep it aside for 10-15 minutes.

Now boil water in a pan or a large pot. Place a stand in the pot on which the tiffin box can be placed.

Keep the amount of water accordingly so that the box is half submerged in water.

When the water boils, place the closed airtight box on the stand and cover the pan/pot with a lid. Keep the flame on medium.

Let the fish steam for 15 minutes in the boiling water.

After that, take the tiffin box out of the water and keep it for 6-8 minutes. This will help the flavors to infuse well.