Prepare the vegetables: Finely chop the cabbage, shred the carrot, and mince the onion. You can use a food processor or knife for a super fine texture. Combine them in a large mixing bowl.
Make the dressing: In a separate bowl, whisk together the mayonnaise, sugar, milk, buttermilk, lemon juice, vinegar, salt, and pepper until smooth.
Combine and toss: Pour the dressing over the cabbage mixture. Stir well to coat all the vegetables evenly.
Chill and serve: Cover and refrigerate for at least 4 hours, or overnight for best flavor. Stir again before serving.
