First, thoroughly wash the fish pieces.
In a large bowl, whisk plain yogurt until it's smooth.
Stir in mustard oil, sugar, ½ teaspoon each of turmeric powder and salt, and ¼ teaspoon of red chili powder.
Mix everything well.
Then, add 2 cups of water to thin the mixture.
Place the Hilsa fish pieces into this yogurt mixture, making sure they are coated on both sides.
Cover the bowl and let the fish marinate for 30 minutes.
Next, combine the remaining turmeric powder, red chili powder, and salt.
Also add the green chilies.
You can prepare Doi Ilish in either a wok or a microwave oven. Both methods are explained below.