In a food processor or blender, combine the garlic, shallots, red chilies, ginger, turmeric, tamarind paste, salt, sugar, and vegetable oil. Blend until you have a smooth paste.
Make several diagonal slashes on each side of the fish to help the marinade penetrate.
Rub the spice paste all over the fish, including inside the slashes and the cavity. Let the fish marinate for at least 30 minutes.
Place each marinated fish on a banana leaf square (or aluminum foil).
Place a lemon slice on top of each fish.
Wrap the fish securely in the banana leaf or foil to form a packet. If using banana leaves, you can secure the packets with toothpicks or kitchen string.
Preheat your grill to medium-high heat.
Place the fish packets on the grill and cook for about 15-20 minutes, turning occasionally, until the fish is cooked through and flakes easily with a fork.
Alternatively, you can bake the fish packets in a preheated oven at 375°F (190°C) for about 20-25 minutes.
Carefully unwrap the fish packets and transfer the fish to a serving plate.
Garnish with fresh cilantro or parsley if desired.
Enjoy your meal!