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.
