In a saucepan, whisk together miso, sugar, mirin, and sake. Cook until thick enough to coat the back of a spoon, about 10 minutes. Transfer to a shallow baking dish. Add fish, turning to coat. Cover with plastic wrap, and refrigerate for 3 days.
Preheat broiler to high. Remove fish from marinade, and place on a baking sheet, skin-side down. Broil until caramelized, about 3 minutes. Using a flat spatula, gently turn fish, and broil until just opaque in center, about 3 minutes more. Serve immediately.