Preheat oven to 350°F. Dry the corned beef brisket with paper towels. Rub the corned beef with mustard on all sides, then sprinkle with the seasoning packet that comes with the meat. Press lightly to adhere. Place onion slices, and smashed garlic in the bottom of a large cast iron dutch oven. Season generously with salt and pepper. Place the corned beef brisket on top of the onion and garlic
Pour beef broth or stock into the dutch oven, followed by the Guinness beer, and Worcestershire sauce. Add in the bay leaves and thyme sprigs. Then cover with a lid. Cook for 1 hour at 350°F, then, without opening the oven door, reduce the heat to 300°F, and cook for an additional 2 hours.
At the 3-hour mark, turn the corned beef over. Check the liquid level, and add more stock, beer, or water if too much has evaporated. Add carrots and potatoes to the pot. Sprinkle with about 2 teaspoons of kosher salt, and 1 teaspoon of coarse ground black pepper. Cook for an additional 1 hour.
At the 4-hour mark, add the cabbage wedges on top, then season generously with salt and pepper. Cover, return to the oven, and cook for 1 more hour.
Let the corned beef rest for 10-15 minutes, and while it's resting. Melt butter in a bowl, add minced parsley, a pinch of salt, and pepper.
Slice the corned beef into thick slices against the grain. Garnish the vegetables with melted butter (optional), fresh parsley, salt, and pepper. Serve with stone-ground mustard. Enjoy!
Store leftovers in an airtight container in the fridge for 4-5 days. To reheat, you can use a microwave, the oven, or the stovetop. Add a bit of broth or water to keep the meat moist and prevent it from drying out. To freeze, all the leftovers to cool at room temperature before transferring them to a freezer safe bag and freeze for up to 3 months. Thaw in the fridge overnight, then reheat.