Preheat the oven to 375 degrees F. Open the packaged corned beef and rinse well. Discard the seasoning packet. Place the corned beef fat side up on a large piece of foil (you will be wrapping the corned beef with the aluminum foil).
Pat the brisket dry with a paper towel. After the brisket is patted dry, spread the mustard over the corned beef and sprinkle with the brown sugar. Add enough water in the pan so the water is 1/2 inch deep.
Place the wrapped brisket in a roasting pan and place in the oven. Cook for about 2 1/2 hours. The internal temperature should be 145 degrees. For super tender corned beef, cook an additional hour or two. Corned beef is extremely tender when the internal temperature is 195.
Open the top of the foil to expose the brisket and turn the oven to broil. Broil long enough for the mustard crust to brown