Remove the 3 lb corned beef brisket with spice packet included from its packaging and rinse it well under cold water. This removes excess surface brine and helps control the saltiness of the final dish. Pat dry with paper towels.

Place the 3 lb corned beef brisket with spice packet included fat-side up in a large Dutch oven. Add the quartered 1 yellow onion quartered, smashed 4 cloves garlic smashed, 2 tbsp whole black peppercorns, 2 whole bay leaves, 1 tbsp brown sugar, and the included spice packet. Pour in the 1 bottle stout beer such as Guinness (if using) and enough 6 cups water or enough to cover the brisket by 1 inch to cover the 3 lb corned beef brisket with spice packet included by about 1 inch.

Bring the pot to a boil over high heat, then immediately reduce the heat to low. Skim off any foam that rises to the surface during the first 10 minutes. Cover tightly and simmer gently — the liquid should barely bubble. Cook for about 2.5 to 3 hours, or until the 3 lb corned beef brisket with spice packet included is very tender when pierced with a fork. A 3 lb corned beef brisket with spice packet included typically takes about 50 minutes per pound.

Once the 3 lb corned beef brisket with spice packet included is tender, carefully transfer it to a cutting board and tent loosely with aluminum foil to rest. Do not discard the cooking liquid — this is your broth for cooking the vegetables.

Bring the broth back to a boil over medium-high heat. Add the 1 ½ lb baby potatoes halved if large and 4 whole large carrots peeled. Cook for 10 minutes.

Add the 1 whole green cabbage core removed wedges to the pot, nestling them into the broth. Cook for another 15 minutes, or until the 1 whole green cabbage core removed is tender but not mushy and the 1 ½ lb baby potatoes halved if large are fork-tender.

While the vegetables finish, slice the 3 lb corned beef brisket with spice packet included against the grain into 1/4-inch slices. Slicing against the grain is essential — it shortens the muscle fibers and makes the meat tender rather than stringy.

Use a slotted spoon to transfer the vegetables to a large serving platter. Dot with 4 tbsp unsalted butter for finishing vegetables if desired and season lightly with 2 tbsp whole black peppercorns (taste before adding salt — the broth is already quite salty). Arrange the sliced corned beef brisket with spice packet included alongside the vegetables, ladle some of the hot broth over everything, and garnish with fresh fresh flat-leaf parsley roughly chopped. Serve with 1 jar whole-grain mustard for serving alongside on the side.