Place the corned beef brisket in the slow cooker, fat side up. Sprinkle the spice packet over the meat.
Add the quartered onion, minced garlic, carrots, and baby potatoes around the brisket.
Pour the beef broth and Worcestershire sauce over the ingredients in the slow cooker. Add black pepper and mustard seeds if using.
Cover and cook on low for 8 to 10 hours, or until the corned beef is tender.
About 1 hour before serving, add the cabbage wedges on top of the brisket and vegetables. Cover and continue cooking until the cabbage is tender.
Once done, remove the corned beef from the slow cooker and let it rest for 10 minutes before slicing against the grain.
Serve the sliced corned beef with the vegetables, garnished with fresh parsley.