In a separate bowl, whisk together the mayonnaise, sugar, milk, buttermilk, lemon juice, vinegar, salt, and black pepper until the mixture is smooth and the sugar has dissolved.
In a large salad bowl, combine the shredded cabbage, chopped carrots, and onion.
Pour the dressing over the cabbage mixture and mix thoroughly until everything is well coated.
Cover the bowl and refrigerate the slaw for at least 2 hours (the longer, the better). Mix again before serving.