Place shredded cabbage, carrots and onions into a medium size bowl.
Add mayonnaise, buttermilk, vinegar, sugar, and salt to a large bowl and whisk together with a fork until well-combined and smooth. Stir until sugar is completely dissolved. Add black pepper and celery seeds. Pour mayonnaise dressing over cabbage and toss together to mix thoroughly.
Serve immediately or cover and chill in refrigerator before serving.
Make-Ahead: Store coleslaw in an airtight container in the refrigerator for up to a week. Vegan: For a vegan version of this coleslaw recipe, use vegan mayonnaise and a plant-based milk.