1 medium sized Hilsa fish, cut into medium sized fillets (approximately 6-8 fillets)
3 tbsp mustard seeds, soaked in water for around an hour
1 medium sized onion, chopped
1 medium sized tomato, chopped (you can replace this with 1 tbsp yogurt)
2-3 fresh green/red chillies, chopped
1/2-inch piece ginger, chopped
1 tsp salt or to taste
1 tsp turmeric powder
Coriander leaves for garnshing
1 tbsp mustard oil (please do not replace it with any other oil. You might skip it as well if you want an oil free version)
Take a clean medium sized microwave safe bowl. Wash and clean the fish and arrange it in the bowl in a single layer. Make sure there is no overlap. Add the salt and turmeric and rub it on both sides of the fish. Keep aside.
Strain the soaked mustard seeds through a sieve for 3-4 times to ensure it is nicely cleaned. In a wet grinder, add the mustard seeds, onions, tomato, ginger, and green chillies (if you are using yogurt, you can add it now). You might add a little water as well. I didn't add any water as the tomatoes were quite juicy. Once the paste is done, pour it into the fish lined out in the bowl. With your hands, mix everything very well so that every fillet gets a fair share of the paste. Add the mustard oil gently over the fish fillets which have been arranged back in place now. Sprinkle coriander leaves. Take a cling wrap and cover the bowl nicely making sure there are no loose ends. Leave it like this for at least an hour (you can skip this step if you are pressed for time; else, if you are getting the dish ready for dinner, you can do it all and just put in the refrigerator and take out 1 hour prior to cooking). About 10-15 minutes before serving the meal, put the prepared bowl into the microwave and cook it on high for 6-7 minutes. Serve hot with plain rice.