Thoroughly rinse the corned beef and discard the seasoning packet. Pat the meat dry.
Place the corned beef fat-side up on a large piece of aluminum foil. Spread the mustard over the corned beef and sprinkle with brown sugar. Wrap the aluminum foil tightly around the meat.
Pour water into a roasting pan until it reaches a depth of 1/2 inch. Place the wrapped corn beef into the pan and roast in the preheated oven for about 2 1/2 hours, until the internal temperature reaches 145°F. For super tender corned beef, cook for an additional 1 or 2 hours, until the internal temperature reaches 195°F.
Carefully open the top of the aluminum foil to expose the brisket and turn the oven to broil. Broil just long enough for the mustard crust to brown.