1) Soak the mustard seeds in 2 spoons water for at least an hour. 2) In your blender, add the soaked mustard, a pinch of salt, and 1-2 green chilies. 3) Add 2-3 spoon water and process to a smooth paste. 4) Clean and wash the fish steaks. Lightly pat dry with paper towel to soak extra water. 5) Use a container with airtight lid, to place the Ilish pieces. 6) Add the mustard paste, turmeric powder, lime juice, and salt to the fish. 7) Pour in most of the mustard oil and thoroughly coat all the fishes with the spices. 8) Place the green chilies on top and spread with remaining mustard oil before closing the lid tightly. 9) In a large pan, bring 1 cup of water to boil over high heat. 10) Carefully place the airtight container in the boiling water and cover the large pan with a lid. 11) Reduce heat to medium to simmer the water. 12) Cook the fish for another 15-20 minutes in simmering water. 13) Then remove it from flame.