Preheat and prep: Heat the oven to 325°F. Set out a large roasting pan. Rinse the corned beef under cold water to remove excess brine, then pat completely dry. Place it fat-side up in the center of the roasting pan.

Make the mustard rub: In a small bowl, stir together the olive oil, whole grain mustard, a generous pinch of sea salt, and freshly ground black pepper. Rub this mixture all over the top of the corned beef, focusing on the fatty side.

Season and braise: Sprinkle the seasoning packet into the bottom of the roasting pan (not on top of the meat). Carefully pour about 4 cups of water into the pan, keeping it away from the mustard-coated surface. Cover the pan tightly with foil and bake for 2 hours.

Prep the vegetables: While the corned beef cooks, prep the vegetables. Core and cut the cabbage into wedges. Peel and wedge the onion. Peel the carrots and cut into large chunks. Halve the potatoes.

Add vegetables and roast: Remove the pan from the oven and increase the temperature to 350°F. Arrange the cabbage, carrots, onion, and potatoes around the corned beef. Spoon some of the hot pan juices over the vegetables. Cover and return to the oven for 30 minutes.

Uncover and caramelize: Remove the foil, spoon more pan juices over the vegetables, and continue roasting uncovered for about 45 minutes, until the potatoes and carrots are fork-tender and the cabbage is beautifully golden and lightly charred at the edges.

Slice and serve: Transfer the corned beef to a cutting board and let it rest for a few minutes. Slice thinly against the grain using a serrated knife. Arrange the sliced corned beef and roasted vegetables on a platter. Finish with fresh parsley and serve warm, with horseradish on the side if you love that extra bite.