Work ahead by steeping the dashi and seasoning the duck the night before or the morning of cooking. For the purest flavor, seek out hon mirin ("real mirin"), which has no added sweeteners or salt.

1 bunch field mustard (nanohana), 500 ml dashi stock (made from dried kombu kelp and dried skipjack shavings), Seasoning A (1/2 cup sake, 1/3 cup sweet mirin sake, 2 to 3 Tbsp soy sauce), 1/2 ...