Add quartered onion to the bottom of the pot.
Put corned beef brisket on top of onions, fat side up. And sprinkle with included seasoning mix.
Lock Instant Pot lid and set pressure to highest pressure and timer to 90 minutes.
Let pressure naturally release or you can do a quick release.
Take brisket out of pot and put on a baking sheet, cover it with tin foil.
Add potatoes to broth in the pot.
Then put the cabbage pieces on top of the potatoes
Set Instant Pot back to highest temperature and timer for 5 minutes.
After timer is done, let pressure naturally release or manually release the pressure
Take cabbage out of pot and put on baking sheet with the corned beef. Cover both with tin foil.
Drain remaining broth and whip potatoes. Add butter and milk.