1. Combine mirin, vinegar, miso, and sugar in a small saucepan. Whisking constantly, cook over medium heat until sugar has dissolved. Remove from heat; transfer glaze to a small bowl and let cool completely.
2. Heat broiler, with rack 6 inches from heat source. Coat a baking sheet lightly with oil. Arrange fish on sheet, and brush generously with miso glaze. Broil until fillets are browned on top and opaque throughout, 6 to 8 minutes.