1 head green cabbage, finely shredded
2 large carrots, finely shredded
3/4 cup best-quality mayonnaise
2 tablespoons sour cream
2 tablespoons grated Spanish onion
2 tablespoons sugar, or to taste
2 tablespoons white vinegar
1 tablespoon dry mustard
2 teaspoons celery salt
Salt and freshly ground pepper