I use mizuna in chicken noodle soup, added at the very end, or in miso soup.
Genus: Brassica rapa, var. nipponsinica
Here's info on mizuna from SpecialtyProduce.com
Dark green leaves offer the most nutrition and provide beta carotene, vitamins, minerals, plus are a source of fiber. Low in calories, greens are a dieter's delight and salvation as they healthy eating without an abundance of calories...
Mix tender little mizuna leaves with other greens or enjoy alone. Steamed mizuna is delicious topped with a light sauce or melted butter. Toss in stir-fries the last minute of cooking. Great in soups. Quickly wilt mizuna leaves; add a touch of garlic and a dash of oil. Mature mizuna makes a perfect substitute for chard or kale...To store, place in a plastic bag; keep in crisper drawer of refrigerator. Do not wash until ready to use; rinse briefly in cold water. For optimum flavor and texture, use within three to five days.