Take corned beef out of the packaging and discard the seasoning packet that came 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.
Cooking Corned Beef:
Preheat the oven to 350°. Tear off a couple of sheets of aluminum foil, long enough to wrap the brisket, and place them in a roasting pan, overlapping like a plus sign.
Slice onions and spread them in the roasting pan, the same shape as the brisket.
Combine the ingredients for the mustard rub and mix them well to combine. Rub corned beef all over with the mixture.
Place meat on top of the onions, fat cap up. Bring aluminum foil together and close it, covering the brisket loosely, leaving a little space between the brisket and the foil.
Place roasting pan into the preheated oven and cook for about 1 1/2 hours.
Add Potatoes:
Take roasting pan out of the oven to add potatoes. Open the foil and nestle potatoes all around the brisket in one even layer so it cooks evenly.
Close the foil again but not too tight. Let everything cook for about 45 minutes.
Open the foil and let it cook for another 15-20 minutes. You can turn on the broiler to low at the end and let the top crisp a little.
Take corned beef brisket out of the pan and onto the cutting board. Tent with a sheet of foil and let it rest for about 10 minutes.
Cut corned beef against the grain and serve with potatoes, onions, and some of the cooking juices.