In Central Africa, both whole fish and fish fillets are cooked in leaf packets over grills or charcoal fires.
Prepare a marinade by mixing together the oil, chopped onion, lemon juice, salt, black pepper, cayenne pepper and any optional ingredients you choose. If you are cooking a whole fish, use a knife to make a few cuts lengthwise on each side. Place the fish in a glass bowl or baking dish. Pour the marinade onto the fish. Let the fish marinate for 15 to 45 minutes in the refrigerator.
Warm the banana leaves for a half-minute in a medium-hot oven, or on a grill, or in a pot of boiling water. This makes them easier to fold. Remove the center rib of each leaf by cutting across it with a knife and pulling it off. Cut the ends off each leaf to form a large rectangle.
Fold the banana leaves to completely enclose the ingredients in a packet two or three layers thick. (Use something like the burrito folding technique. Depending on how many leaves and how much (or how many) fish you are cooking you may want to make more than one packet. Use oven-proof string to tie them closed.)
Cook the packets over an outdoor grill, or in an oven. (If using an oven, you may want to place some aluminum foil under them to catch drips.) Turn them every ten minutes. After 30 minutes carefully open the packet and check to see if the fish is cooked, if it is not, close the packet and continue cooking.
Servings 4