Chef’s note: Banana leaves from your own garden—or store-bought squares of banana leaves, which come frozen in plastic bags that are easily defrosted—may be used to wrap a variety of foods. If the banana leaves are dry, soak them for a half-hour or so in water and shake out. You may also substitute aluminum foil or parchment paper. As for the herbs, this is a laissez-faire recipe, with no strict requirements. Use handfuls of lemongrass, fennel, and other herbs if you have them. If, however, all you have is a small jar of lemongrass, use what you have! The same thing is true of the fragrant Kaffir lime leaves. They can be added or left out entirely.
Preheat oven to 400°F. Spread out banana leaves on baking sheet, overlapping them, removing any thick veins, and pounding them down with your fists or a small mallet to have them lie evenly. Chop lemongrass, fennel, herbs, and Kaffir lime leaves. Mix herbs with butter or oil and spread half on top of banana leaves on baking sheet. Season mixture with salt and pepper, if desired. Oil exterior of fish and lay on top of chopped herb mixture. Stuff some additional herbs into cavity of fish. Top with remaining chopped herb mixture.
Wrap banana leaves up around fish, securing with lemongrass or other ovenproof ties. If using parchment paper or aluminum foil, create a loose packet, crimping or folding top edges to secure. (This may be done up to eight hours in advance. Refrigerate if preparing more than an hour ahead of time.)
Bake for approximately 10 minutes, or until thermometer registers 135°F. Serve unwrapped on a platter.