Step 1: Make the spice paste: Place the macadamia nuts, if using, ginger, turmeric, shallots, garlic, chiles, and black pepper in a food processor and grind to a fine paste, running the machine in short bursts. (You can also pound the ingredients to a paste in a heavy mortar with a pestle.) Work in the fish sauce.

Step 2: Heat the oil in a wok over medium-high heat. Add the spice paste and fry it until dark and fragrant, 4 to 6 minutes, stirring with a wooden spoon. Transfer the spice paste to a plate or bowl and let cool to room temperature. Using a spoon or spatula, thickly spread each piece of fish on both sides with spice paste.

Step 3: Set up your grill for direct grilling and heat to high. If using fresh banana leaves, grill on each side for 10 seconds to soften the leaf. Skip this step if using frozen leaves. Lay a banana leaf square, dark side down, on your work surface. Place a piece of fish in the center. Fold over one side, then the other, to encase the fish. Then pin each end with a toothpick to seal the package. Each package will be about 7 inches by 4 inches. The fish packages can be prepared several hours ahead and refrigerated until grilling.

Step 4: Arrange the banana leaf packages, smooth side down, and grill until nicely browned, 3 to 5 minutes. Turn the packages over and grill the same way—total cooking time should be 6 to 10 minutes. To test for doneness, insert a metal skewer through the side of one packet and leave it for 15 seconds: it should come out very hot to the touch.

Step 5: Serve the fish in the banana leaf. Let each eater open his own package.