Combine the sake and mirin and bring to a boil to burn off the alcohol, 5 minutes. If you waft your hand over the pot toward your nose and feel a slight burning sensation, there’s still some alcohol left.
Add the sugar and mix until fully dissolved, then add the miso and stir until combined. Remove from the heat.
Transfer to a cup container to cool to room temperature or cool over a bowl set over ice. Label the container “Miso Marinade” with the date and store it in the fridge for up to a year.
Cook everything, stirring continuously with a spatula, except miso marinade over low heat for 10 mins in a small saucepan until very soft.
Mix in the miso marinade and cool completely in the fridge before serving with white rice.
