Cut each banana leaf into half. Mix mustard powder, turmeric powder, red chilli powder, salt, mustard oil and green chillies and apply this paste evenly on the fish pieces.
Place three to four banana leaves on a tawa and then put another set of leaves on top crosswise. Carefully place the hilsa pieces on top and put the remaining masala and oil on top. Fold the banana leaves on the fish pieces and tie with a string. Place tawa over high heat and cook.
Colour of the leaves will start changing from green to yellow. It will take twenty to twenty five minutes for hilsa to cook. It is always better to cook the fish just before serving.