Sent by AimeeEditor: Aimee, right now my favorite thing to do with bitter greens or other fresh greens from the garden is to chop them roughly and toss them with boiled new potatoes, olive oil, salt and pepper. It really mellows out spicy greens but still allows their flavor to come through.
For other ideas for mizuna specifically, check out Kathryn's great post on Asian greens:
Readers, what do you like to do with mizuna?
Related: Ingredient Spotlight: Mustard Greens