1. Center an 18- by 28-inch piece of foil (or overlap two 12-inch-wide pieces) in a 14- by 17-inch baking pan. Rinse banana leaf and pat dry; center lengthwise on foil. Rinse fish and pat dry; rub your fingers over the flesh to find any bones, pull out with tweezers, and discard. Center fillet lengthwise, skin down, on leaf.
2. Mix the cilantro, thyme, and oregano. In a bowl, mix half the herb mixture with crab, mayonnaise, and mustard. Spread evenly over salmon. Rinse and dry bowl.
3. Rinse squash and bell pepper. Trim and discard stems from squash; stem and seed pepper. Cut vegetables into matchstick-size strips (about 3 in. long). Put vegetables, wine, olive oil, 1/4 teaspoon salt, and rest of herbs in bowl and mix to coat.
4. Spread vegetable mixture over crab layer. Bring long sides of foil together over fish, wrapping banana leaf partially around fish, then fold in ends to seal.
5. Bake in a 450° regular or convection oven until fish reaches 140° in center of thickest part (insert thermometer through foil and vegetable and crab layers to test), 20 to 25 minutes. Transfer packet to a platter and unwrap to serve; or, if desired, with two wide spatulas, carefully transfer leaf and fish from foil to platter. Add salt and pepper to taste. Cut fish into portions; slip a spatula between skin and fillet and lift off portions, leaving skin behind.