Place the trivet in the bottom of the Instant Pot. Add diced onions and garlic cloves on top.
Rinse the corned beef under cool water and place it fat side up on top of the onions.
Pour in 3 cups of beef broth and stir in the spice packet. Make sure there’s enough liquid to cover about halfway up the brisket.
Secure lid, set sealing position, and cook on high pressure for 80 minutes.
Allow natural pressure release for 20 minutes, then carefully release the remaining pressure.
Remove the beef roast with a slotted spoon and place it on a cutting board. Cover loosely with foil to rest.
Strain the cooking liquid, setting onions and garlic aside. Return 2 cups of the broth to the pot.
Add cabbage wedges, carrots, and potatoes. Season lightly with salt and pepper.
Secure lid again and cook for 5 minutes on high pressure. Perform a quick release.
Slice corned beef against the grain and serve with vegetables. Ladle rest of the broth over the top for best results.