Rinse the corned beef under cold water to remove excess brine. Place it in the slow cooker, fat side up. Sprinkle the provided spice packet over the meat.
Add the onion, garlic, bay leaves, and peppercorns around the brisket. Pour in enough beef broth to just cover the meat (if using beer, add beer first and then use beef broth to cover meat).
Cover and cook on LOW for 7 hours or on HIGH for 4 hours. Add the potatoes and carrots during the last hour of cooking.
Nestle cabbage wedges around meat in the slow cooker. Cover and cook on LOW for another 1 hour (or on HIGH for 30 minutes), until the cabbage is tender.
Carefully remove the corned beef and let it rest for 10 minutes before slicing against the grain. Drizzle with some of the cooking liquid and serve warm with mustard and parsley.
If you opt for beer, pour 6 oz of beer before adding beef broth and cover the beef completely. Proceed with remaining steps.
Notes
Use a flat-cut brisket for easier slicing and even cooking. Don’t skip rinsing the brisket—it prevents excess saltiness. Layer in the vegetables so they stay tender, not mushy.