Remove the outer leaves from the cabbage and cut it into quarters. Remove the core from each quarter. Using a sharp knife or mandoline slicer, finely shred the cabbage into thin strips. Place the shredded cabbage in a large mixing bowl. Grate the carrots using a box grater and add them to the cabbage along with the finely chopped sweet onion.

In a separate medium bowl, whisk together the mayonnaise, milk, buttermilk, sugar, apple cider vinegar, lemon juice, salt, black pepper, and celery seed until the sugar is completely dissolved and the dressing is smooth and creamy.

Pour the dressing over the cabbage mixture and toss thoroughly until all the vegetables are evenly coated. Cover the bowl with plastic wrap and refrigerate for at least 2 hours, or preferably overnight.

Before serving, give the coleslaw a good stir to redistribute the dressing. Serve chilled as a side dish with your favorite main courses.