In a bowl,combine together all the ingredients listed under'For marination'.Rub this onto the fish until evenly coated.Set aside for about 15 to 20 minutes.
Heat coconut oil in a pan,add the marinated fish and fry over medium low flame for about 2 to 3 minutes on both the sides.This is just to make the fish firm.Set aside until required.
Meanwhile heat oil in a pan (you could also use the same pan in which you have fried the fish )add the ginger,garlic,green chilies,curry leaves and shallots.Cook over medium flame until shallots turn golden brown.Add salt to speed up caramelisation.
Reduce the flame to low,add turmeric powder,coriander powder,black pepper powder ,kashmiri chili powder and 1/4 cup hot water.Cook ,stirring continuously ,until the masala starts to leaves the sides of the pan.
Now add the chopped tomato and cook over medium flame until it turns soft and mushy.
Next add the coconut milk and stir to combine.Add vinegar ,fenugreek powder and more salt as required ,mix well.Cook for few more minutes or until the gravy turns thick.Switch off the flame and allow to cool slightly.
Heat the banana leaf over low flame ,to make it pliable.Drizzle few drops of coconut oil on the banana leaf and spread 2 tbsp of the prepared masala on top.
Place the fried fish on top and spread 2 tbsp of the masala to evenly coat the fish.
Place few curry leaves and green chilies on top.Drizzle few drops of oil and fold the leaf from all the sides to make a parcel.Tie it with a banana fiber or a string.Similarly wrap the remaining fish.
Heat a heavy bottomed pan,add 2 tsp oil and place the fish parcels and cook over medium heat for about 8 to 10 minutes on both the sides.Serve immediately with lemon wedges,sliced tomato and onion.