Preheat the oven to 350 degrees F and place a large piece of aluminum foil in an oven-proof dish with high sides like a Pyrex dish.

Remove the corned beef from the package. Set the spice packet aside. Throw out any other packaging. Rinse the corned beef with cold water, pat dry, and set it on the foil in the dish, with the fatty side on the top.

Rub ¼ cup of mustard all over the corned beef (top, bottoms, sides).

Open the seasoning packet, and sprinkle the seasoning on top of the mustard-rubbed corned beef. The seasoning only needs to go on the top of the meat, not all over.

Tightly wrap the corned beef with the foil it's resting on. Place pan with the foil wrapped corned beef into the oven. Bake for 1 hour per pound of corned beef.

Once the beef has cooked, remove it from the oven and open the foil.

Optional: Broil the corned beef for 5 minutes, or until the top begins to crisp.

After all cooking (broiling or not) has completed, let the corned beef rest for 10 minutes before transferring it to a cutting board. Slice the meat against the grain and serve with sautéed cabbage.