good amount of marinade for 4 - 6 fish servings (about 1 1/2 - 2 pounds)
Bring mirin and sake to boil in a small saucepan over medium heat. Boil for 20 seconds then remove from heat and mix in miso, sugar and sesame oil. Pour over fish filets in a shallow glass baking dish. Turn fish filets to coat with marinade. Cover and refrigerate for at least 2 (and up to 6) hours.
Preheat broiler with oven rack 6-inches from heat source. Line a broiler pan with a sheet of foil and oil the foil. Remove fish from marinade and place on foil-lined pan. Broil fish until opaque in center, about 6 minutes.