In a medium bowl, combine sake, mirin, miso, sugar and soy sauce and whisk to combine. Place fish and sauce in a large sealable plastic bag and marinate in the refrigerator for at least 2 hours and up to 6 hours.

Preheat broiler. Remove fish from marinade and place on a greased baking sheet. Broil on high for 8 to 12 minutes or until just opaque in the center and lightly golden on top. Transfer to serving plates.