3 pounds small (1 1/2-to 2-inch) Japanese turnips with greens
1 1/3 cups water
2 tablespoons mirin (Japanese sweet rice wine)
Stir together miso and 2 tablespoon butter. Discard turnip stems and coarsely chop leaves. Halve turnips (leave whole if tiny) and put in a 12-inch heavy skillet along with water, mirin, remaining tablespoon butter, and ⅛ teaspoon salt. Bring to a boil over medium-high heat, then boil, covered, 10 minutes.
Add greens by handfuls, turning and stirring with tongs and adding more as volume in skillet reduces. Cover and cook 1 minute. Uncover and continue boiling, stirring occasionally, until turnips are tender and liquid is reduced to a glaze, about 5 minutes. Stir in miso butter and cook 1 minute.