In a small bowl comine the milk, buttermilk, vinegar, and lemon juice. If using the pineaple juice and horseradish, mix those in as well.
Sprinkle the xanthan gum slowly into the liquid mixture while whisking vigorously to prevent clumping. Then let rest 2-5 minutes
While waiting, finely chop or grate the cabbage and carrots to your desired consistency, then combine in a large bowl.
In a medium sized bowl, whisk together the sugar, salt, pepper, and mayonnaise. Once combined, whisk in the milk and vinegar mixture.
Pour the dressing over coleslaw and onion mixture; mix well.
Cover and chill at least one hour.