Marinate the fish
In a medium bowl, mix together the sake, mirin, miso, brown sugar, soy sauce and ginger until smooth and blended. Place Alaska sablefish fillets and marinade in a large zip-top bag; squeeze out air and seal. Place the bag in a shallow bowl or baking dish. Refrigerate at least 12 hours or up to 72 hours, turning the bag over occasionally to coat all sides. Remove fish from marinade; drain thoroughly.
Make the cucumber salad
In a bowl, combine cucumbers, carrots, and cabbage. In a smaller bowl, whisk together the ginger, sesame oil, rice vinegar, sugar, soy sauce and sriracha until the sugar is dissolved. Pour dressing over vegetables; sprinkle with sesame seeds and toss to combine. Cover and refrigerate until serving, up to 8 hours.
Broil fish and serve
Turn oven on to broil setting. Place fish in an oven-proof pan. Arrange oven rack so that top of fish is 4-5 inches from broiler heat element. Broil 4-5 minutes, or until top of fish is caramelized and dark brown; remove and allow fish to rest 2-3 minutes. To serve, portion about 2 cups cucumber salad on each plate. Top sablefish with 1/2 tablespoon scallions.