Toss the cabbage and the onion with 2 tablespoons of the salt in a large colander and set aside over another bowl or in the sink until the vegetables have wilted, about 1 hour. Rinse the cabbage mixture thoroughly under cool running water and then spin it dry in a salad spinner.

While the cabbage is wilting, make the dressing: Whisk the vinegar, sugar, mustard, celery seeds, pepper, and remaining ½ teaspoon kosher salt together in a small nonreactive mixing bowl. Little by little, add the oil in a slow, steady stream to form a smooth, slightly thickened dressing.

When you are ready to serve, toss the drained, wilted cabbage mixture with the green onions and enough of the dressing to coat the slaw. Taste and adjust the seasoning if necessary.