Scatter the garlic and onions in the bottom of the crockpot. Place beef on top of onions and garlic.
In a cup, mix together water, beer, sugar and vinegar. (If you are not using beer, use water in its place.)
Pour over beef and then sprinkle the mustard seeds, bay leaves, allspice berries and pepper over the beef.
Cover the slow cooker and cook on low for 5 hours. Add the carrots, potatoes and cabbage to the slow cooker and stir, then cover and set to high. Cook for another 2 hours. Remove the beef; once it has cooled, slice across the grain into 1/2-inch pieces.
Taste the broth and season to taste (the corned beef may have salted it enough).
To serve, place the sliced beef in the hot cabbage broth and let it warm through. Serve warm.