Prep the Veggies: In a large bowl, combine finely chopped cabbage, grated carrots, and minced onion. Toss to mix evenly.
Make the Dressing: In a separate bowl, whisk together mayonnaise, buttermilk, milk, lemon juice, vinegar, sugar, salt, and black pepper until smooth.
Combine: Pour the dressing over the cabbage mixture and stir until everything is thoroughly coated.
Chill: Cover and refrigerate for at least 2 hours (preferably overnight) to let the flavors develop.
Serve: Give it a quick stir before serving. Best served cold and fresh!