Preheat the oven to 325°F (163°C).

Soak the corned beef: Submerge the brisket in cold water for about 15 minutes. This helps rinse away surface brine and prepares the meat for even cooking. After soaking, drain well and pat dry with paper towels.

Prepare the baking dish: Place the brisket (fat‑side up if your cut has it) into a baking dish. Pour in 1 cup of stout beer around (not over) the meat.

Mix the crust rub: In a small bowl, combine the dark brown sugar, whole grain mustard, and the seasoning packet from the corned beef. Stir until the sugars are moistened and the mixture is uniform.

Apply the rub: Spread or rub the sugar‑mustard mixture evenly over the top surface of the brisket.

Cover & bake: Cover the dish tightly with aluminum foil or a lid and bake in the preheated oven for 4 hours. For cuts larger than 4 pounds, add about 1 additional hour per extra pound.

Finish uncovered: For the final 30 minutes, remove the cover so that the top can brown and develop a crust.

Rest & slice: Remove the corned beef from the oven. Allow it to rest for about 10 minutes before slicing—this helps retain juices and makes slicing cleaner. Slice against the grain into thin slices.