01 - Rinse the corned beef brisket under cold water and pat dry. Place it in the bottom of a large slow cooker fat-side up.
02 - Sprinkle the included spice packet contents (or mustard seeds, if using) evenly over the brisket.
03 - Arrange the onion quarters, smashed garlic, carrots, and potatoes around the sides of the brisket in the slow cooker.
04 - Pour in the beef broth and water until the meat and vegetables are barely covered. Add bay leaves and black peppercorns to the liquid.
05 - Cover and cook on LOW setting for 8 hours, or until the beef is fork-tender and easily pulls apart.
06 - During the final hour of cooking, carefully tuck the cabbage wedges into the slow cooker around the beef and vegetables.
07 - Remove the corned beef from the slow cooker and let rest for 10 minutes before slicing thinly against the grain.
08 - Plate the sliced corned beef with cooked vegetables, ladling some of the cooking broth over each portion if desired.