Remove the corned beef brisket from the package and discard the spice packet.
Place the brisket in a pot of water and bring to a boil, then immediately remove it to reduce excess salt.
Preheat the oven to 350°F.
Place the brisket fat side up on a large piece of aluminum foil.
Rub the top and sides of the brisket evenly with Dijon mustard.
Sprinkle brown sugar over the mustard-coated brisket and gently pat it on.
Wrap the brisket loosely with the foil, leaving some space inside, and place it in a rimmed roasting pan.
Bake the covered brisket for 2 hours, adjusting time for larger cuts (about 45-60 minutes per pound).
Uncover the brisket, turn on the oven broiler (low setting if available), and caramelize the top for a few minutes, watching carefully to prevent burning.
Remove the brisket from the oven and let it rest for 10-15 minutes before slicing against the grain.