To prepare the marinade, heat the sake and mirin together on high heat to a boil for 20 seconds or so to remove the alcohol. Then turn the heat down to low and add the miso paste, using a whisk or wooden spoon to incorporate. Then turn the heat back up and add the sugar stirring constantly until it all dissolves, then remove from heat and let it cool. The whole process will take only 2 – 3 minutes stove time, at most.
Once cooled, put the fish in a pan and slather it with the cooled miso sauce. Cover and refrigerate for at least 48 hours. Food and Wine says you can do it in only 24. I haven’t tried it but I’m sure it would still be fabulous.
To cook the fish, wipe off the marinade (if you don’t it will burn too quickly because of the sugars). Now you can choose to either grill it quickly or broil it. I put it in a pan, broil it until the surface browns, then just turn the oven to bake at either 400 degrees conventional or 375 degrees convection for about 5 minutes. The recipe says 10 – 15 but I find that with the broil method, it needs only a short time baking to be flaky and ready to eat. Of course, this will depend on the thickness of your fish. The butterfish I buy is thin. If you find fish that is thicker, it will take longer.