Add onion, carrots, celery and potatoes to bowl of your crock pot.
Place corned beef on top and season generously with salt, pepper, and pickling spices.
Scatter thyme and bay leaves over meat and fill slow cooker bowl with water* until it is almost completely covering the meat.
Cook on low for 9 to 10 hours, or on high for 4 to 4 1/2 hours, until meat is tender.**
Add cabbage to crock pot and cook on high for 45 minutes to 1 hour more until cabbage is tender and cooked through.
Remove meat and vegetables from the liquid in the slow cooker. Let the meat rest for 10 minutes before removing the fat cap off the top, slicing and serving.
Notes
* Feel free to use a combination of water and your favorite ale.**The low heat is recommended for the most tender brisket but if you're short on time the original recipe claims that the high heat for the shorter amount of time works, just not as well.