Take a large salad bowl out. Put the shredded carrots and cabbage into it and toss well so that both vegetables are evenly distributed.
In a small mixing bowl, whisk together the mayonnaise, sour cream, onion, sugar, vinegar, mustard, celery salt, salt, and pepper. Mix them thoroughly.
Pour the mayonnaise dressing over the carrot and cabbage mixture. Toss well so that the vegetables are coated. Taste for the seasoning. You can vary the amount of salt and pepper according to preference. Some people even add sugar to make the whole affair sweeter. Serve immediately. The vibrancy of the colors makes it simply irresistible.