Add the onion to the bottom of your instant pot. Place the corned beef on top and sprinkle with pickling spice.
Add the water the instant pot and make sure the valve is turned to seal. Set the instant pot to high and cook for 80 minutes.
Once the time is up, carefully turn the valve to release and let the remaining pressure out. Remove the beef and onions from the instant pot and set it it aside to rest.
Take out two cups of the water and add the carrots, potatoes, and cabbage. Make sure the valve is turned to seal. Turn the pressure cooker to high for 5 minutes. Once the time is up then release the pressure. Once the pressure is released slice up the meat going against the grain and serve with the veggies.