Prep the Beef
Rinse the corned beef brisket under cool water to remove excess brine, then pat dry. Place it fat-side up in a large Dutch oven or slow cooker, and sprinkle the spice packet over the top. Add bay leaves, garlic, and peppercorns.
Add Liquid
Pour in enough water (and beer, if using) to cover the beef. The liquid helps tenderize and infuse flavor throughout the meat.
Simmer Until Tender
Bring to a boil over high heat, then lower to a gentle simmer. Cover and cook 2 ½–3 hours, or until fork tender.
If using a crock pot, cook on Low for 8–9 hours or High for about 5 hours.
Add the Vegetables
Add carrots, potatoes, and onions once the beef is nearly tender. Cook for 20 minutes, then add cabbage wedges and cook another 15 minutes until vegetables are soft but not mushy.
Rest and Slice
Transfer beef to a cutting board, rest 10 minutes, then slice against the grain.
Serve
Arrange beef and veggies on a platter, spoon over some broth, and top with butter or fresh parsley if desired.