Preheat the oven to 325 degrees F. Set out a large roasting pan. Rinse the corned beef brisket in cold water, then pat dry. Place it in the roasting pan.
In a small bowl, mix the whole grain mustard, brown sugar, and black pepper. Rub the mixture over the top of the brisket. (The fatty side.)
Sprinkle the seasoning packet in the base of the roasting pan. Pour 4 cups of water into the pan, being careful not to wash the mustard off the brisket. Then cover the top of the pan tightly with foil. Bakes for 2 hours.
Meanwhile, cut the cabbage in half and remove the core. Cut each half into 4 wedges. Peel the onion and cut into wedges. Trim and peel the carrots. Then cut them into 2-3 inch segments.
When the corned beef comes out of the oven, adjust the temperature to 350 degrees F. Place the garlic cloves in the roasting pan juices. Then arrange the cabbage, onion, carrots, and potatoes around the brisket.
Spoon some of the pan juices over the vegetables. Then place back in the oven and bake for 30 minutes, uncovered. Then spoon more broth over the veggies and bake for another 30 minutes, or until the largest carrots and potatoes are fork-tender.
Use a serrated knife to slice the corned beef brisket into thin strips. Then arrange the corned beef and vegetables on a platter and serve warm.
Serve with a side of extra mustard or creamy horseradish sauce.Leftover Corned Beef and Cabbage is delicious! Keep cooled leftovers in an airtight container in the fridge for 3-4 days.