Place the trivet or steamer rack in the bottom of the Instant Pot and set the corned beef brisket on top, fat side up.
Sprinkle the spice packet over the brisket, then add the quartered yellow onion and smashed garlic cloves around the beef.
Pour in the beef broth and water, ensuring the liquid comes up around the beef without fully covering it.
Secure the lid, set the vent to sealing, select Pressure Cook (High), and cook for 90 minutes.
Allow a natural pressure release for 15 minutes, then perform a quick release of any remaining pressure and open the lid.
Carefully transfer the cooked corned beef to a cutting board, tent with foil to keep warm.
Add the halved red potatoes and carrot chunks to the remaining liquid in the Instant Pot, secure the lid, and cook on High Pressure for 4 minutes; then quick release.
Add the cabbage wedges to the pot, secure the lid, and cook on High Pressure for 2 minutes; then quick release.
Slice the corned beef against the grain and arrange on a platter with the potatoes, carrots, and cabbage. Ladle some cooking liquid over top and serve with Dijon mustard or horseradish if desired.