Gather the ingredients.
Whisk mayonnaise, sugar, milk, buttermilk, lemon juice, vinegar, salt, and black pepper in a separate bowl until smooth and sugar has dissolved.
Mix cabbage, carrots, and onion in a large salad bowl.
Pour dressing over cabbage mixture and mix thoroughly.
Cover bowl and refrigerate slaw at least 2 hours (the longer the better). Mix again before serving.
