Gather all ingredients.
Place half of the yellow onion, potatoes, and carrots in the bottom of an 8-quart slow cooker.
Place corned beef on top of vegetables, fat side up, and sprinkle with seasoning packet or mixed pickling spice.
Place remaining vegetables around and on top of corned beef. Pour beer or broth around corned beef.
Cover and cook on LOW 7 to 8 hours.
Add cabbage wedges to slow cooker and let cook on LOW 2 hours longer or until cabbage and corned beef are tender.
When ready to serve, remove vegetables from slow cooker and transfer to a serving platter. Thinly slice or shred corned beef and add to platter with vegetables. Sprinkle with parsley (if using).
