Preheat oven to 325 degrees Fahrenheit
Rinse the brisket under cold water then pat dry.
Arrange the brisket in a large roasting pan or dutch oven. The fat cap should be placed upwards.
Spread 2 tablespoons dijon or yellow mustard over the top.
Sprinkle the seasoning packet on top of the mustard then sprinkle with 2 tablespoons brown sugar.
Add enough Guinness beer, beef broth or water to the pan so that it is about halfway up the corned beef brisket.
Cover with foil or place a lid on the pan and bake for two hours.
After two hours, add in potatoes, carrots, onion, and cabbage. Place the cover back on the pan and return to the oven for 90-120 minutes, or until the corned beef brisket has reached an internal temperature of around 180 degrees.
Remove the veggies from the pan, then broil for 3-5 minutes. This is optional but adds beautiful color to the corned beef brisket.
Allow the corned beef to rest for 10 minutes, then slice into pieces making sure to cut against the grain.
Serve with potatoes, carrots, cabbage and onion.
*Optional - serve with horseradish sauce - see note above about how to make it.