First, make the Maui Onion Relish. In a nonreactive mixing bowl, stir together all the relish ingredients. Cover with plastic wrap and refrigerate for 2 hours. Preheat the oven to 400 degrees F. Lightly grease or oil a baking dish large enough to hold the fish fillets side by side.
Evenly spread the chopped macadamia nuts on a large dinner plate. Season a fish fillet with salt and pepper, and brush it evenly on both sides with 1/4 of the butter mixture. Dredge it in the nuts to crust it evenly on both sides, and transfer to the baking dish. Repeat with the remaining fillets, butter mixture, and nuts.
Bake the fish fillets until they are cooked through and the nut crust is golden brown, 8 to 10 minutes. Spoon the Maui Onion Relish onto 4 serving plates and, with a spatula, transfer a fillet to each plate, placing it on top of the relish. Serve immediately.
Prepare the Ginger Butter by putting all the ingredients in a food processor with the metal blade and processing until smooth. Set aside at room temperature.;