Cut fish into eight 4-ounce pieces. Place fish in one 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.