Stir Together The Onion, Garlic, Mustard, Vinegar, Bay Leaves, Peppercorns, Salt, Parsley, And Celery Seed In A Bowl. Cover, And Refrigerate For 24 Hours. Rub The Brisket With The Mixture, Wrap Tightly, And Refrigerate Overnight. To Cook The Brisket, Place It Into A Slow Cooker Along With The Water. Cover, And Cook On Low For 5 Hours. Add The Carrots, And Cabbage, And Continue Cooking Until The Brisket Is Tender, About 3 Hours More.