Japanese mustard spinach. Brassica rapa var. perviridis, コマツナ、小松菜。
A variety of Brassica rapa, the plant that also has given us the turnip and Mizuna. Although it resembles spinach somewhat, the plant is more like a leafy turnip.
A pure Japanese vegetable, komatsuna has been cultivated in Japan since olden times. It gets it name from Komatsu-gawa in Tokyo, where it was originally harvested in the Edo-period. Komatsuna was offered to the Shogun Yoshimune when he was on a falcon hunt in the area.
Komatsuna is originally a favorite winter vegetable (although now harvested throughout the year). The glossy leaves are rich in calcium. The leaves can be harvested at any stage of growth; the flavor grows stronger the more the leaves mature. Major growth areas are around the big cities: Tokyo, Saitama, Chiba and Kanagawa; Osaka and Hyogo; and Aichi and Fukuoka.
There are many ways to use this versatile vegetable: in nabemono (one-pot dishes), in soups, in ohitashi (soused greens), stir-fried (as itamemono), boiled and even pickled. It can also be used in salads. In fact it can be used in any way spinach is used. In the Kanto area, komatsuna is also used in the New Year's dish of zoni.