Miso is a fermented soybean paste. The lighter, or "white," version is milder and less salty than brown miso, and is sometimes referred to as sweet miso. It can be found in the Asian section of some supermarkets and most health-food stores.
In a small saucepan, combine mirin, miso, and sugar. Whisking constantly, cook over medium heat until sugar has dissolved. Remove from heat, and let cool completely.
Heat broiler, with rack 6 inches from heat. Coat a baking sheet with oil. Place fish on baking sheet, and brush liberally with miso mixture. Broil until fillets are browned on top and opaque in the center, 6 to 8 minutes. If tops brown before fish is cooked through, cover loosely with aluminum foil. Remove from oven; serve immediately.