Begin by finely chopping the green cabbage, grating the carrot, and mincing the yellow onion. Combine in a large bowl.

In a separate bowl, whisk together mayonnaise, sugar, and whole milk until smooth. Add buttermilk, apple cider vinegar, lemon juice, salt, and pepper, and mix well.

Pour the dressing over the chopped vegetables and stir until well-coated.

Cover the bowl and refrigerate for at least two hours to allow flavors to meld.

Before serving, stir the coleslaw gently and adjust seasoning if needed. Serve chilled.