Trefoil, Japanese wild chervil. 三つ葉、みつば。(Cryptotaenia japonica).
The name means literally "three leaves," and indeed, one leaf of mitsuba consists of three smaller ones. The deeply-cut leaves are attached to slender green stalks.
[Mitsuba. Photo Ad Blankestijn]
Mitsuba is related to parsley and celery. The taste is like mild chervil. Mitsuba is nicely fragrant.
Used in soups (miso soup), egg custards, salads and nabemono. Mitsuba is harvested in spring, but available the whole year.