Preheat oven to 350°F. Pat the corned beef brisket dry with paper towels. Rub stone-ground mustard all over — top, bottom, sides, everything. Sprinkle the seasoning packet over the mustard and press lightly to adhere. Place sliced onions and smashed garlic in the bottom of a large Dutch oven (6-7 quart) and season generously with salt and pepper. Set the corned beef on top of the onion and garlic, mustard side up.
3 pounds corned beef brisket, 3 tablespoons stone-ground mustard, 1½ tablespoons corned beef seasoning
Pour in the beef broth, followed by the Guinness and Worcestershire sauce. Tuck in the bay leaves and thyme sprigs. The liquid should come about halfway up the sides of the brisket — if it doesn't, add a splash more broth. Cover with a tight-fitting lid and place in the oven. 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.
2 medium onions, 8 cloves garlic, 1 teaspoon kosher salt, ½ teaspoon coarse ground black pepper, 4 cups beef stock, 1 bottle Guinness stout, 3 tablespoons Worcestershire sauce, 3 whole bay leaves, 3 sprigs thyme
At the 3-hour mark, carefully flip the corned beef over. Check the liquid level and add more broth, beer, or water if it has dropped significantly. Add the carrots and baby potatoes to the pot, nestling them into the braising liquid around the brisket. Sprinkle kosher salt and pepper over the vegetables. Cover and return to the oven for 1 more hour.
5 large carrots, 2 teaspoons kosher salt, 1 teaspoon coarse ground black pepper, 1.5 pounds baby red potatoes
At the 4-hour mark, add the cabbage wedges on top. Season generously with salt and pepper. Cover and return to the oven for 1 final hour.
1 medium cabbage
Transfer the brisket to a cutting board. Tent loosely with foil and let it rest for 10-15 minutes. While the beef rests, melt butter in a small bowl and stir in minced parsley, a pinch of salt, and pepper.
Slice the corned beef into thick slices against the grain. Drizzle the parsley butter over the vegetables (optional). Serve with stone-ground mustard. Enjoy!
½ cup unsalted butter, 2 tablespoons parsley, salt & pepper to taste