1/2 cup well-shaken buttermilk
2 tablespoons mayonnaise
2 tablespoons cider vinegar
2 tablespoons minced shallot
1 tablespoon sugar
3 tablespoons finely chopped chives
1 pound Napa cabbage, cored and thinly sliced crosswise (4 cups)
6 radishes, diced
2 celery ribs, thinly sliced diagonally
Whisk together buttermilk, mayonnaise, vinegar, shallot, sugar, 1/2 teaspoon salt, and 1/4 teaspoon pepper in a large bowl until sugar has dissolved, then whisk in chives.
Toss cabbage, radishes, and celery with dressing.