Make the dressing: add the apple cider vinegar, mirin, miso, sesame oil, honey, garlic, and shallot to a food processor or blender and process until smooth.

Rinse the quinoa well in a fine-mesh sieve under running water. Add it to a saucepan with the water and bring to a boil. Cover, reduce the heat to its lowest setting, and cook for 12 minutes. Turn off the heat and leave covered for 10 minutes, then fluff with a fork. Spread onto a plate or shallow bowl and refrigerate until fully chilled.

Toast the walnuts in a dry frying pan over low heat, shaking the pan occasionally, for about 5 minutes or until fragrant. Remove from the heat and let cool completely.

Add the celery, apple, chickpeas, watercress, and celery leaves to the chilled quinoa and toss to combine. Transfer to a serving bowl, scatter the toasted walnuts on top, and drizzle with the dressing. Toss just before serving.