1. Preheat the oven the 325°F. Set one oven rack in the middle position and another in the bottom position.
2. Rinse the corned beef several times under running cold water. (No need to dry it.)
3. Place the corned beef fat side up in a large roasting pan (you'll trim the fat after the meat is cooked). Pour about ⅛ inch water around the meat. Sprinkle the contents of the seasoning packet into the water around the corned beef. Cover the pan tightly with heavy-duty aluminum foil and roast on the middle rack for 3 hours.
4. Meanwhile, in a small bowl, mix together the melted butter, horseradish, salt and pepper.
5. Place the carrots, potatoes and cabbage side by side on a rimmed baking sheet (do not line the pan with foil; the potatoes will stick). Drizzle the horseradish-butter mixture over the veggies and toss with a spatula to coat all of the vegetables evenly, keeping the vegetables separate. Turn the potatoes so that they are cut side down (they'll get crispier that way).
6. After the corned beef has roasted for 1 hour and 30 minutes, place the pan of vegetables on the bottom rack of the oven. Roast the vegetables and continue cooking the corned beef for 1 hour and 30 minutes more, until both the meat and vegetables are tender. Check on the veggies occasionally to be sure they are browning evenly. (The cabbage will brown first, so give it a toss when the bottom pieces look golden. The potatoes and carrots may need to be turned, but only if they are nicely browned on the bottom before the cook time is up.)
7. Transfer the corned beef to a cutting board and let sit until cool enough to handle, about 5 minutes. Cut off the layer of fat on top of the corned beef and discard. Slice the meat against (or perpendicular to) the grain into ¼-inch slices. Arrange the meat on a platter with the roasted vegetables and sprinkle with parsley, if desired. Serve with the horseradish cream sauce.

For the Horseradish Cream Sauce

1. Combine all of the ingredients in a small bowl and stir to combine. Taste and adjust seasoning, adding more horseradish to taste, if desired.
2. Note: You won't use the whole head of cabbage; just use enough to cover ⅓ of the sheet pan.
3. Note: The nutritional information does not include the horseradish cream sauce.