Cut onion in large chunks and place in bottom of a 6 quart slow cooker. Place corned beef on top. Add spice package if one was included with the corned beef.
Pour broth (or water) overtop and add garlic and bay leaves.
Cook on low for 3 hours and then add potatoes and carrots. Cook for an additional 5-7 hours, adding the cabbage wedges in the last 2 hour of cooking.
Remove corned beef from slow cooker and let rest 15 minutes before slicing. Serve with potatoes, carrots and cabbage on the side.
Notes
To reduce cooking time, simmer the corned beef in the crock pot set on "low" for three house. Then raise it to "high," add the potatoes, carrots, and cabbage wedges, and cook for one hour more.
Be sure to let the corned beef rest before slicing, to allow the juices to seep back into the meat.
To cut cabbage into wedges, use a large, sharp knife. Trim the stem, then with the stem facing up, cut cabbage head in half, bisecting the stem. Then cut each half into thirds through the stem to ensure the leaves hold together.