You may have seen mizuna, a member of the mustard family, at Asian and health food markets. Indigenous to Japan, the green has both a long growing season and a high tolerance for cold weather, which makes it easy to grow.
Mizuna's taste is peppery like arugula and slightly bitter like frisee, yet it's milder and sweeter than either of the more commonly found salad greens. Mizuna is usually not eaten raw in Japan—instead, it's pickled, stir-fried, simmered, and added to hot pot dishes.
With its crisp stalks and beautiful frond-like leaves, mizuna is a wonderful addition to salads, especially where frisee is normally used, such as the classic frisee aux lardon. Mizuna is not only cheaper than frisee, but you can eat the whole thing—the stems are mild and sweet. With frisee, you have to trim the bitter green ends.
When mizuna is sauteed, it retains its juiciness and sops up much of the flavoring liquid you may use, such as stock or soy sauce. Try it with a bit of ponzu or lemon, both of which complement the slight bitterness of the leaves.