Using a mortar and pestle or food processor, blend all the ingredients except the fish, banana leaves and lemon basil until a smooth paste forms.
Make slits on both sides of the fish and spread evenly with the paste.
Place the fish on the banana leaf and lay the lemon basil leaves on top. Fold into a parcel, securing the ends with toothpicks.
Either steam the fish for about 30 minutes, or barbecue over charcoal for about 10 to 15 minutes on each side, until the leaf edges are getting charred. Slowly open the parcel to check--if the fish is opaque and the flesh flakes easily with a fork, it is cooked.