Cabbage and Celery Root Slaw with Cider-Dijon Dressing Recipe

1/2 medium head green cabbage, thinly sliced (about 8 cups)
1/2 small head red cabbage, thinly sliced (about 4 cups)
2 red bell peppers, cut into matchstick-size strips 3 cups matchstick-size strips peeled celery root (from 1 medium head)
2/3 cup mayonnaise
2 tablespoons Dijon mustard
2 tablespoons apple cider vinegar
4 teaspoons sugar
2 teaspoons celery seeds

Mix first 4 ingredients in large bowl. Whisk mayonnaise and next 4 ingredients in small bowl to blend. Pour dressing over vegetables. Season with salt and pepper. (Can be made 6 hours ahead. Cover; chill.)