Prepare the banana leaf which will be used for baking the fish. Clean the banana leaf and dry it with a kitchen towel. Pass the leaf over low heat to make the leaf pliable.

Next, mix together the fish pieces, half of the chopped onions, lemon juice and green chilies, mustard seed paste and salt in a large bowl. Put the fish along with the mixed ingredients on the banana leaf and pack it into a parcel. Secure the parcel with a string.

Place the secure parcel on a tawa over medium heat. Roast on both sides till the aroma of the baked fish starts coming out. Each side will take about 10-15 minutes to cook.

Once the parcel is opened, add the remaining chopped onions and the mustard oil to the baked fish. Mash everything together using your hand or a fork.