In a medium saucepan over medium heat, combine the sake, mirin, miso, soy sauce and sugar. Stir until everything is well combined and the sugar is disolved. Remove from the heat and cool to room temperature.

Pat the fish fillets thoroughly dry with paper towels. Put them in a lock-top type plastic bag and pour in the miso sauce. Refrigerate for at least 2 and up to 8 hours.

Preheat the oven to 400°F. Preheat a grill or broiler. Gently shake fish to remove any excess miso clinging to the fillets. Place the fish on the grill or under the broiler and cook about 4 minutes per side until its surface turns brown. It may blacken in spots. The fish should be just opaque in its center.

Arrange the fish on individual plates or a platter and garnish with the green onions.