Preheat water bath to 140°F.
In a small saucepan over medium heat, combine sake, mirin, miso, and sugar. Cook until well combined and slightly thickened, about 5-8 minutes. Remove from heat and set aside.
Cut fish into even serving portions (2 pounds serves 4-6 people).
Place each piece of fish into a freezer-grade Ziploc bag (the fish is delicate so you don't want to use a vacuum sealer).
Divide the sauce evenly into each bag and use the water displacement method to seal. (At this step you can leave the fish in the miso mixture in the fridge for up to 2 days for a stronger miso flavor or cook immediately for a more mild flavor).
Cook at 140°F for 45 minutes.
Carefully remove fish from bag, saving all of the bag liquid.
Dump all of the bag liquid into a large pan over medium-high heat. Very carefully add the fish (it's delicate!) and allow to cook in the miso mixture until caramelized, about five minutes.