In a 1-cup measure stir together soy sauce, mirin, and sake and reserve 1/4 cup. In a small saucepan simmer remaining 3/4 cup soy sauce mixture and sugar until reduced to a glaze (about 1/4 cup) and remove pan from heat.
Using tweezers, remove any bones from sea bass. Trim spongy root ends from mushrooms. Have ready a shallow baking pan lined with paper towels. In a 12-inch nonstick skillet heat oil over moderately high heat until it just begins to smoke and sauté fillets, skin sides down, pressing gently with a spatula if fillets curl, until skin is golden, about 3 minutes. Turn fillets over and sauté 2 minutes more, or until almost cooked through. Add reserved (uncooked) soy sauce mixture to fish and simmer until fish is just cooked through, about 1 minute. Transfer fillets, skin sides down, with a slotted spatula to baking pan to drain and add mushrooms to liquid remaining in skillet. Cook mushrooms, stirring frequently, until tender, about 1 minute, and remove skillet from heat.
Arrange fish and mushrooms on 6 plates. Brush fish with glaze and garnish with chives.
