Instructions Preheat the oven to 350°F. Place a 12×12-inch piece of foil in the center of a large baking dish.In a bowl, combine the mustard, honey, brown sugar and onion powder. Set aside ¼ cup of this the sauce for serving.Discard the seasoning packet included with the corned beef. Pat the brisket dry with paper towels and place it in the pan on the foil, fat-side-up. Use a paring knife to cut slits in the top, spaced 1-2 inches apart.Spoon the honey mustard mixture over the top of the brisket. Top with another piece of foil and pinch a seam around the bottom foil to seal. Transfer the pan to the oven to bake for 2 hours, or until the beef reaches an internal temperature of 160°F.Remove the top foil and broil for 3-4 minutes, or until the top is golden brown and crispy (optional).Allow the brisket to rest for at least 10 minutes. Cut into ¼-inch-thick slices against the grain. Serve with the reserved honey mustard.