Place the onion pieces at bottom of the inner pot of the Instant Pot.
Place brisket with as much of its juices as you can on top of the onions.
Add the seasoning packet to the top.
Add broth around the brisket, being careful not to dislodge any of the seasoning.
Set timer or pressure cook setting for 90 minutes (high or manual, depending on your Instant Pot or settings).
Quick release the Instant Pot once the brisket is finished cooking.
Remove brisket and onions from the Instant Pot and place into a bowl and cover to keep warm.
Add vegetables to the broth in the Instant Pot along with the olive oil.
Season with salt and pepper – to taste and seal lid.
Set to pressure cook high or manual cook for 4 minutes.
Quick release and remove the lid.
Serve corned beef brisket and vegetables together, immediately.
Spoon leftover juices from the Instant Pot over the meat and vegetables if desired.