Also known as Japanese mustard, mizuna is considered a specialty mustard green. Its feathery serrated leaves have a much tamer flavor than standard mustard greens or even arugula.
These delicate leaves (and stalks too) are mild enough to be eaten raw in a salad or a top a noodle soup. They can also be sautéed, steamed, or cooked into a stir-fry or batch of mixed greens.
Mizuna can also serve as a substitute for mustards, collards, and kale in most recipes. Add the greens to a soup or stir-fry in the last few minutes of cooking so they are just wilted when served. Or, take off the thick stem and mix the tender leaves into a fresh salad (It is excellent mixed with sweeter items like pomegranate seeds or apple slices).
Store your greens, dry and unwashed, in a plastic bag in the fridge. They should last about a week.