Marinated for three days before cooking the fish, you make the four-ingredient marinade (sugar,sake,miso and sugar) and then seal it in a covered container in the fridge. Keep an eye on it so it doesn’t get dried out as the fish leaches water out over the marinade and you need to take out the water each day.
Make the miso marinade and marinate the fish by bringing the sake and mirin to a boil in a medium saucepan over high heat. Boil for 20 seconds to evaporate the alcohol. Turn the heat down to low, add the miso paste, and whisk. When the miso has dissolved completely, turn the heat up to high again and add the sugar, whisking constantly to ensure that the sugar doesn’t burn on the bottom of the pan. Remove from heat once the sugar is fully dissolved – cool to room temperature.