Rinse brisket with cold water and thoroughly pat dry.
Place onion slices into a 6 or 8-quart Instant Pot. Place the brisket on top of the onion mixture. Sprinkle with brown sugar, garlic, and pickling spices. Pour in beer and beef broth around the corned beef.
Place the lid on the pressure cooker and be sure the vent knob is sealed. Select the manual or pressure cook setting and set the cooking time for 85 minutes on high pressure.
Once the cooking time has elapsed, let the pressure release naturally for 15 minutes. Then do a quick release of any remaining pressure.
Remove the corned beef from the Instant Pot and place it on a serving platter. Cover with foil to keep warm and let the meat rest.
Remove the onions from the pressure cooker using a slotted spoon. Drain off all but 2 cups of the cooking liquid in the Instant Pot.
To the inner pot, add the potatoes and carrots and top with cabbage. Season with salt and pepper, to taste.
Place the lid back on the pressure cooker, be sure the vent knob is sealed, and cook on high pressure for 5 minutes. Once the cooking time has elapsed, let the pressure release naturally for 5 minutes then do a quick release of any remaining pressure.
Thinly slice corned beef against the grain and serve with potatoes, carrots, and cabbage.
Brisket Choice: This recipe works with both a flat cut or point cut, and no changes to time need to be made. The flat cut is leaner, while the pointcut is more marbled. Beer: Feel free to use any dark stout beer you like, or beef stock if you do not drink beer. The beer adds a great amount of flavor but is not necessary.Pickling Spices: Most packaged corned beef brisket comes with a pickling spice blend. If yours does not, you can use 1 tablespoon of generic pickling spices. Gluten-Free Corned Beef and Cabbage: If you want this to be a gluten-free meal, check your corned beef label carefully to be sure the brisket and seasoning packet does not contain gluten. And use an additional 1 cup of beef stock in place of the stout beer.Potatoes: You can use red potatoes, russet potatoes, or new potatoes. Cut your potatoes into 2-inch chunks or leave them whole if using baby potatoes.Leftovers will store in the refrigerator for up to 3 days. You can freeze the leftover corned beef brisket for up to 3 months. Please be sure to refer to Instant Pot High Altitude Conversion if needed.