Place, carrots, onions, and potatoes into a slow cooker.
Place corned beef (fat side up) on top of vegetables.
Sprinkle with pickling spice.
Add enough beer to almost cover the beef.
Cover and cook on high for 6.5 hours.
Add cabbage, and replace lid.
Cook for another 1.5 hours until corned beef is tender.
Serve with yellow mustard.
