Add poppy seeds, mustard seeds, coconut, green chilies, and 1/4 cup water to a grinder and grind to as smooth a paste as possible.
Add coconut marinade to cleaned fish fillets, add salt, and mustard oil and let sit for 30 minutes.
Lightly heat the banana leaves over an open flame. This will make the leaves more pliable and easy to fold. Cut banana leaves into required number of pieces making sure they are large enough to hold the individual fillets.
Arrange a fish fillet on a banana leaf, drizzle some mustard oil, add a few strips of lime leaves, and one whole green chili. Fold as shown in the video or use kitchen twine to tie the leaves.
Heat oil in a thick bottomed pan. Arrange the folded paturis making sure not to crowd the pan. Fry on one side for 5-6 minutes. Flip and fry on the other side for 3-4 more minutes. You can alternately steam the paturis for 20 minutes using a double boiler.