How to Make It
In a small saucepan, bring the mirin and sake to a boil. Whisk in the miso until dissolved. Add the sugar and cook over moderate heat, whisking, just until dissolved. Transfer the marinade to a large baking dish and let cool. Add the fish and turn to coat. Cover and refrigerate overnight.
Preheat the oven to 400°. Heat a grill pan and oil it. Scrape the marinade off the fish. Add the fish and cook over high heat until browned, about 2 minutes. Flip the fish onto a heavy rimmed baking sheet and roast for 10 minutes, until flaky. Transfer to plates and serve with pickled ginger.