Wash and clean the fish pieces in cold water. Pat it dry with a kitchen towel. Drizzle some mustard oil over the fish pieces and rub them with a little salt and 1/2 tsp turmeric so that the fish is coated well on both sides. Keep it aside.
Combine the mustard seeds, poppy seeds and the grated coconut and dry grind it in a mixer or food processor.
Add the lemon juice, green chilli, turmeric, salt and grind it into a smooth paste by adding very little water(about 1/4 cup). The mustard paste is ready.
Take out the mustard paste in a medium size bowl. Add 1.5 cup of water or as needed, salt as per taste, 4 tablespoons mustard oil, and paprika into the bowl.
Mix well or blend all the ingredients in a blender for few seconds. Strain the mixture into a medium size bowl and discard the residue left in the strainer. You will get a brightly colored mustard sauce.
Add salt and red chilly powder into the mustard sauce if needed.
Grease a microwave oven safe container with mustard oil and place the marinated fish in a single layer and pour the mustard sauce over the fish,add the remaining green chilies,drizzle some oil on the top if you wish.
Cover the container with a clingfilm. Cook in microwave at high for 5 minutes, take it out, remove the cover and stir the sauce.
Cover again with a clingfilm, put it back in the microwave and again cook for 5 minutes. Take it out again, remove the cover, stir the sauce and flip the fish to the other side.
Once again , cover the container and cook in the microwave for another 4-5 minutes or until the fish is completely baked. Do not overcook. The fish in bengali mustard sauce is ready.
Please note that the cooking time may slightly vary with the power of the microwave, so adjust accordingly.
Serve the Microwave Steamed Fish in Bengali Mustard Sauce with hot white rice.