Bring sake and mirin to a boil in a medium saucepan over high heat. Boil for 20 seconds to evaporate alcohol.
Reduce heat to low and add miso paste, mixing with a wooden spoon. When miso has dissolved completely, increase heat to high and add sugar, stirring constantly to ensure bottom of pan does not burn. Remove from heat once sugar is fully dissolved. Cool to room temperature. Set aside ½ cup of sauce for plating the next day, and use the rest as marinade.
Pat fish thoroughly dry with paper towels. Cover with saikyo-miso sauce and place in a glass dish or bowl and cover tightly with plastic wrap. Refrigerate overnight.
Preheat broiler to high.
Lightly wipe off and excess miso clinging to fillets, but do not rinse it off fish. Discard marinade. Place fish in a broiler pan, and broil until surface if fish turns brown, about 2 minutes.
Reduce temperature to 400°F.
Move fish to center of oven and bake 10 to 15 minutes.
Transfer fillets to individual plates. Add a few drops of reserved sauce to each plate.
Marinate this dish overnight. The sauce also works well with beef and salmon.