2-3 days before serving, make the miso marinade and marinate the fish. Bring the sake and mirin to a boil in a medium saucepan over high heat. Boil for 20 seconds to evaporate the alcohol. Turn the heat down to low, add the miso paste and whisk. When the miso has dissolved completely, turn the heat up to high again and add the sugar, whisking constantly to ensure the sugar dissolves completely and doesn’t burn on the bottom of the pan. Once the sugar is dissolved, remove from heat and cool to room temperature.
Pat the fish fillets thoroughly and dry with paper towels. Slather the fish with the miso marinade and place in a non-reactive dish or bowl and cover tightly with plastic wrap. Put in refrigerator and marinate for 2 – 3 days.
To Cook the Fish: Preheat oven to 400F. Heat an oven-proof skillet over high heat on the stovetop. Lightly wipe off excess miso clinging to the fish fillets. Don’t need to rinse off. Film the pan with a little oil, then place the fish skin-side up in the pan and cook about 3 minutes, until the bottom of the fish browns and blackens. Flip the fish and continue cooking until the other side is browned, 2-3 minutes. Transfer to the oven and bake for about 5 minutes, until the fish is opaque and flakes easily.
Instead of cooking on the stovetop, you can also use the grill or broiler. For the grill: place the fish skin-side down. For the broiler: place on oiled foil that lines a sheet pan, skin-side up. For both, have about 6 inches from heat. Cook 2-3 minutes on each side until brown and blackened on each side. If necessary, can finish in a 400 F oven for about 4-5 minutes. Fish is done when it pulls apart easily with a fork.
If short on time, can shorten the marinating time down to 3 hours but the complexity of the flavor won’t be as strong.