Brush both sides of the corned beef with stoneground mustard. Place corned beef in a 9 x 13inch baking dish with the fat side up. Sprinkle with seasoning packet. Pour both Guinness beers around the corned beef.
Cover tightly with aluminum foil and bake at 325 for 2 hours. Uncover and bake for an additional 45 minutes - 1 hour or until an internal temperature reaches between 195 - 205 degrees F. Remove from oven and allow to rest 10 minutes before slicing.
Slice against the grain and serve.
Notes
We are cooking the brisket fat side up so that as it cooks, it helps keep the meat tender as it cooks. It will also give you that perfectly crisp topping that we are looking for.Brisket will be the most tender when cooked to a temperature between 195-205 degrees F. Letting the meat rest 10 minutes before slicing is essential. It allows the juices to redistribute and ensures a more tender bite. Nutrition information for estimation purposes only.