Slice the leek into thin strips, soak in water, and drain. Wipe the kombu with a dry cloth if dirty and cut into 3cm pieces. Soak kombu in water for 1 hour.

Remove the skin and tail from the sardines, then chop them finely until sticky. Mix the sardines with the tsukune seasoning ingredients in a bowl.

Heat the kombu water over low heat, remove the kombu before boiling, then add bonito flakes and let them sink for 5–6 minutes. Strain the broth using a sieve and kitchen paper.

Heat 400ml of the strained dashi broth, drop tsukune into the broth, and simmer for 5 minutes while skimming off foam. Add salt and soy sauce to taste.