In a large mixing bowl, combine shredded cabbage, grated carrots, and diced onions.
In a separate bowl, whisk together mayonnaise, milk, buttermilk, sugar, vinegar, lemon juice, salt, and pepper until smooth.
Pour the creamy dressing over the vegetable mix, tossing everything well to coat.
Cover the bowl and refrigerate for at least 2 hours to let the flavors meld.
Stir the coleslaw before serving to refresh its consistency and chill.