1. Prepare banana leaves: Gently pass fresh banana leaves over a low flame or dip in hot water for a few seconds until pliable. Wipe clean and pat dry. Cut into 4-6 equal portions, each large enough to wrap a serving of fish or shrimp.

2. Prepare marinade: In a medium bowl, combine the thinly sliced red onion, diced tomato, julienned bell pepper, chopped cilantro, minced garlic, lime juice, olive oil, turmeric (if using), red pepper flakes (if using), salt, and black pepper. Mix well.

3. Marinate seafood: Add the fish fillets and shrimp to the marinade. Gently toss to ensure all pieces are well coated. Let marinate for 15-20 minutes at room temperature.

4. Assemble packets: Lay one portion of banana leaf flat. Place a few slices of unripe green banana or plantain in the center. Top with one fish fillet or a portion of shrimp from the marinade, ensuring some vegetables are included.

5. Wrap securely: Fold the long sides of the banana leaf over the filling, then fold the ends to create a neat, secure packet. You can tie with kitchen twine or a thin strip of banana leaf to keep it closed if desired.

6. Cook the packets: Arrange the banana leaf packets in a steamer basket over simmering water. Steam for 15-25 minutes, or until the fish is opaque and flaky, and the shrimp are pink and cooked through. Cooking time may vary based on the thickness of the fish and the size of the shrimp. (Alternatively, the packets can be baked at 375°F (190°C) for 20-30 minutes or grilled over medium heat for 8-12 minutes per side).

7. Serve immediately: Carefully unwrap the packets and serve the aromatic fish and shrimp with their cooked vegetables. Enjoy with a side of steamed jasmine rice or sticky rice.