In a large mixing bowl, combine the coleslaw mix, shredded carrots, and finely chopped onion. Toss lightly to mix the vegetables evenly.
In a separate bowl, whisk together the mayonnaise, buttermilk, milk, sugar, lemon juice, and white vinegar until the mixture is smooth and fully combined.
Pour the dressing over the cabbage mixture. Use a large spoon or spatula to toss everything together until all the vegetables are coated evenly with the dressing.
Season with salt and pepper to taste. Mix again to incorporate.
Cover the bowl with plastic wrap and refrigerate for at least 2–3 hours before serving. This allows the flavors to meld and the cabbage to soften slightly while staying crisp.
Serve chilled as a side dish with fried chicken, sandwiches, or your favorite barbecue meals.
Enjoy!