Open the foil packet carefully—hot steam will escape. Spoon any accumulated juices into the bottom of the pan, but leave the brisket sitting on top of the foil, still fat-side up and now uncovered.
Return the baking sheet with the unwrapped corned beef to the 400°F (200°C) oven. Roast for 15–20 minutes, or until the fat cap is bubbling, browned, and crisp around the edges, watching closely so the sugar in the glaze doesn’t burn.
Remove the pan from the oven and let the corned beef rest, uncovered, on the baking sheet for at least 10–15 minutes. This rest helps keep the meat juicy when you slice it.
Transfer the brisket to a cutting board and slice it against the grain into thin or medium-thick slices, depending on how you like it. Spoon some of the pan juices over the slices before serving for extra moisture and flavor.
For a slightly sweeter, more caramelized crust, sprinkle an extra teaspoon or two of brown sugar over the top of the mustard glaze before sealing the foil packet. If you prefer a bit of heat, use spicy brown mustard or add a pinch of crushed red pepper flakes to the brown sugar and mustard mixture. You can also adjust the cooking time slightly for texture: if you like your slices very firm for sandwiches, pull the brisket a little earlier when it’s just tender enough to slice cleanly; for fall-apart meat, let it go closer to the upper end of the time range. If your corned beef is very lean, lay a small sheet of parchment directly on top of the brisket before wrapping in foil to help hold in moisture. For easier cleanup and a more rustic look, you can roast two smaller briskets side by side on the same foil-lined sheet—just check them a bit earlier, as they may cook faster. Leftovers freeze well: wrap sliced corned beef tightly in foil, then in a freezer bag, and reheat gently in a low oven, still wrapped, to keep it juicy.
