Put the chicken into cold water, bring to the boil and simmer for about 15 minutes. Leave to cool slightly, lift out of the water and shred the flesh into thick strips.
Combine the miso, mirin, sugar, dashi and sake in a pan, slowly stirring the liquid into the miso with a whisk. Simmer for 20–30 minutes until thick and caramelised. Add mustard and vinegar. Take off the heat and stir in the chicken. Sprinkle with spring onions