Preheat the oven to 325°F (165°C). Line a large rimmed baking sheet with foil or parchment paper.
Place the potatoes and carrots on the baking sheet and drizzle with 1 tablespoon of olive oil. Toss to coat and spread evenly.
Place the corned beef brisket in the center of the sheet pan, fat side up.
Sprinkle the seasoning packet evenly over the corned beef.
Arrange the cabbage wedges around the corned beef and vegetables. Drizzle with the remaining olive oil or melted butter.
Cover the entire sheet pan tightly with foil to trap moisture.
Roast for 1 hour and 45 minutes, or until the corned beef is fork-tender.
Remove the foil and return the pan to the oven for an additional 15 minutes to lightly brown the vegetables.
Remove from the oven and let the corned beef rest for 10 minutes before slicing against the grain.
Serve warm with mustard or horseradish if desired.