Wash fish fillets. Sprinkle salt, turmeric and red chilli powder and refrigerate for about 15 minutes.
Mix together mustard paste, poppy seeds paste, coconut paste, turmeric, salt, onion seeds, red chilli powder, sugar and 1 tbsp of mustard oil.
Coat the fish fillets with the above paste and keep aside for 30 minutes.
Cut the banana leaves into (8x6 inch rectangles). Soften the cut leaves by dipping in warm water. Wipe dry.
Apply mustard oil on a banana leaf. Place a marinated fish fillet on the leaf and put a green chilli on top. Fold the leaf into a parcel and tie up with thread. Do the same for all the fish fillets.
Heat a frying pan and add remaining oil to it. Place a banana leaf on top, such that the base of the pan is covered. Place the fish parcels on the leaf and cook covered on low-medium heat for 10 minutes. Flip over and cook the other side for 10 minutes. (You can also steam the fish in a steamer for about 20 minutes till cooked through or bake the fish at 350F for 20-25 minutes).
Remove, and transfer the banana wrapped fish fillets onto a serving dish.
Unwrap Bhetki Macher Paturi and enjoy the delicious flavour with Steamed Rice.