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°F.
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.
BAKE TIME: 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.
Video
Notes
Why boil first? Since corned beef is cured in salt brine, it contains quite a bit of salt. So bringing the corned beef to a boil in a pot of water, gets some of the saltiness out.
Adding potatoes and/or cabbage: You absolutely can add some potatoes and if you wish cabbage and carrots to the brisket. Since beef will take MUCH longer to cook than vegetables, you will need to cook the brisket for a while first. Baby gold potatoes will need about an hour of total cook time, covered and uncovered. So set aside about an another hour at the end of meat’s cooking time to cook potatoes. If your potatoes are larger, you can always cut it in half. If you wish to add cabbage, cut the head of cabbage into 4-6 wedges and place them around the meat. Depending on the size of the cabbage wedges, it would take 45-60 minutes. You can see the steps and get the idea of how to cook it in my Baked Corned Beef Brisket and Potatoes recipe.
Store and reheat: Store leftover cooked brisket in an air-tight food storage container, in the refrigerator. Properly stored, it should be good for 3-4 days. To reheat the sliced of corned beef brisket, I recommend doing it in the oven to preserve the juiciness. Wrap some slices in foil and reheat in the oven at 350°F just until heated through.