Take the brisket out of the package and discard the spice pack that comes with it.
Place brisket in a pot of water and bring it to boil. As soon as it starts to boil, take the brisket out. This will remove some of the excess salt in the meat.
Preheat the oven to 350°.
Place the brisket, fat layer up, in the middle of aluminum foil big enough to cover the whole brisket.
Rub the whole brisket with Dijon mustard. Then, evenly sprinkle brown sugar over the top and gently pet it on. Pat some onto the sides as well.
Bring the aluminum foil together and close it, covering the brisket loosely, leaving a little space between the brisket and the foil. Place the covered brisket into a rimmed roasting pan.
Bake it for 2 hours. Open the foil and turn on the broiler on low, if you have an option for low or high broiler setting. (If not, move the brisket to a lower rack.) Let the top caramelize for a few minutes but watch it so it doesn't burn.
UPDATE: the 2 hours recommendation time is for the 1.75-2 lb. sized corned beef brisket. If you have a larger piece of meat, you will need to cook it longer. Give it about 45-60 minutes per pound. Lean towards the 60 minutes per pound time.
Take the corned beef brisket out and let it rest for a 10-15 minutes on the cutting board before slicing.
Make sure to slice the meat against the grain! Luckily, with the corned beef brisket, it's very easy to see the grain.