Combine the sake and mirin in a saucepan. Bring to a boil over high heat. Reduce the heat to medium-low. Add sugar. Simmer, stirring, until the sugar dissolves. Cook, without stirring, for a further 2 minutes or until reduced by half.
Whisk miso into the pan until the glaze is well combined and coats the back of a spoon. If the glaze is too thin, simmer for a l