01 - Rinse the corned beef brisket under cold water to remove excess brine. Place in a large stockpot and add 12 cups water to cover.
02 - Add the spice packet contents, bay leaves, peppercorns, mustard seeds if using, garlic, and onion. Bring to a boil over high heat, then reduce to a gentle simmer. Skim off any foam that rises.
03 - Cover and simmer for 2.5 hours, or until the beef is tender when pierced with a fork.
04 - Add the potatoes and carrots to the pot around the beef. Continue simmering for 15 minutes.
05 - Add the cabbage wedges, submerging as much as possible. Simmer for an additional 20 minutes until all vegetables are tender.
06 - Remove the brisket and let rest for 10 minutes before slicing across the grain.
07 - Arrange sliced beef and vegetables on a serving platter. Ladle a little cooking liquid over the top before serving.