Marinade the hilsa pieces with salt and 1 tsp turmeric powder.
Soak both the mustard seeds in half a cup of water for 15 minutes.
Discard the water and take the mustard seeds in a grinder jar. Add a couple of green chillies, 1 tbsp of mustard oil and little water for grinding it into a smooth paste. It takes some time to make a good paste and add more mustard oil or water, if required to make the paste.
In a frying pan or a kadai, take 1.5 tbsps of mustard oil. When it heats, add a couple of green chillies and then nigella seeds. Let them splutter. Then add the rest of the turmeric powder and the mustard paste. While you are doing so, keep the heat to a medium to low.
Stir this for a few seconds and then add water. Add as much water as the desired consistency.
Let the water come to a boil and then transfer the fish into the mustard gravy.
Keep the heat at a medium and let it simmer for 3 to 4 minutes. Then flip the fish and let the other side cook for 3 - 4 minutes.
Then drizzle some more mustard oil and add more split green chillies and let it boil.
Season with salt if required and once oil starts to rise, turn off the heat.
Serve shorshe ilish with steamed rice.