Cut fish into 8 (4-ounce) pieces. Place fish in 1 layer in a shallow dish. Pour 2 cups den miso over fish, turning to coat thoroughly. Cover with plastic wrap, and marinate in refrigerator for 2 to 3 days, turning once a day. Heat broiler to high. Remove fish from marinade, and place on a baking sheet, skin side down. Broil until caramelized, 2 to 3 minutes. With a flat spatula, turn fish, and broil 2 to 3 minutes more. Serve immediately.
Combine miso and sugar in the top of a double boiler. Add sake and mirin, and whisk to combine, using a heavy whisk. Cook over boiling water, stirring frequently until sugar is dissolved and the color begins to darken, 30 to 45 minutes. Remove from heat, and cool.
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