In large bowl, combine cabbage/carrot mix and onion. In separate smaller bowl, make dressing by combining sugar, salt, pepper, celery seed, milk, mayonnaise, buttermilk, vinegar and lemon juice, beating until smooth. Pour dressing over cabbage mixture and stir to coat. Cover and refrigerate for at least 2 hours, then stir before serving. If you like a drier coleslaw, before stirring, drain off some of the dressing that collects at the bottom of the bowl after refrigeration.