1/4 cup buttermilk
2 tablespoons fresh lemon juice
1 tablespoon mayonnaise
3 tablespoons olive oil
Kosher salt and freshly ground black pepper
1 head of iceberg lettuce outer leaves removed, thinly sliced
1/2 small head green cabbage outer leaves removed, thinly sliced
4 scallions thinly sliced
3 tablespoons chopped fresh chives
1 teaspoon celery seeds
Whisk buttermilk, lemon juice, and mayonnaise in a large bowl, then whisk in oil.
Season dressing with salt and pepper.
Add lettuce, cabbage, scallions, chives, and celery seeds to dressing and toss to coat well.
Taste and season with salt and pepper.
DO AHEAD: Iceberg, cabbage, scallions, and chives can be sliced 2 hours ahead. Chill in a large bowl, covered with a damp paper towel.