Add onion and garlic to the bottom of a Calphalon 7-Qt. Digital Slow Cooker. Place brisket on top of onion and garlic, then add water, beer or ale, and corned beef spice packet. Depending on size of brisket, you may need to add additional water to cover brisket. Set crock pot to cook on low setting, cover, and cook for 8 hours.
After 4 hours of cooking time, add cabbage, carrots and potatoes. Re-cover and continue cooking.
Serve with spicy mustard and horseradish (optional).