Rinse or pat the meat with a paper towel to remove some of the brine
Add the onion to the pot, and place the corned beef on top
Spread mustard over the beef, then sprinkle the seasoning packet over the beef and into the pot
Pour the beer into the pot, and add water
Cover, and turn valve to Sealing position, and hit PRESSURE COOK for 75 minutes (115)
Pot will take a few minutes to reach pressure then begin counting down and cook. Allow a 15 minute release, then turn valve to Venting to quick release remaining pressure
When pin drops, carefully open lid, and remove beef to a large platter. Cover with foil
Remove some of the cooking liquid, leaving at least 1 cup for the 6 quart or 2 cups for the 8. Place the trivet or steamer basket into the pot, and add potatoes, carrots and cabbage
Add lid, turn valve to sealing, and hit PRESSURE COOK for 4 minutes
When cook time is up, do a quick release. When pin drops, remove lid and carefully remove vegetables. Arrange on a platter with the corned beef, and serve