Unfold banana leaf. Cut off and reserve the tough center rib that runs along the bottom edge of each leaf with kitchen scissors.
Cut ribs into 12- to 14-inch strips for tying packets. Gently cut leaves into eight 12-inch squares, being careful not to split them.
Slowly run a banana-leaf square across flame of a gas burner, just enough to "melt" outer coating on leaf - it will become quite shiny. If you have an electric stove, hold leaf slightly above burner on medium-high setting.
Place a filet of sea bass in center of square and spread achiote paste over top and bottom of the fish.
Assemble seven more packets in same manner and set aside to season for at least two hours.
Bake fish packets in middle of oven 20 minutes and then transfer each to a large plate. Serve with pickled red onions.
ONIONS: In a non-reactive bowl, mix all ingredients. Let onions marinate in refrigerator for at least 24 hours before using.