Remove the outer leaves from the cabbage and cut it into quarters. Remove the core, then finely shred the cabbage using a sharp knife or mandoline slicer. Place the shredded cabbage in a large mixing bowl. Add the shredded carrots and minced onion, then toss everything together to combine evenly.

In a medium bowl, whisk together the mayonnaise, buttermilk, white vinegar, sugar, lemon juice, salt, pepper, and celery seed. Continue whisking until the sugar has completely dissolved and the dressing is smooth and creamy.

Pour the dressing over the cabbage mixture and toss thoroughly to coat every strand. Cover the bowl with plastic wrap and refrigerate for at least 2 hours (preferably 4-6 hours) to allow the flavors to meld and the cabbage to soften slightly.

Before serving, give the coleslaw a good stir. Taste and adjust seasoning if needed. Serve chilled as a side dish with your favorite BBQ, fried chicken, or sandwiches.