Place corned beef brisket in the Instant Pot and sprinkle with pickling spice. Top with garlic and onion and pour in water. Place the lid onto the pot ensure the vent is sealed. Press the "Meat/Stew" button and then the "Adjust" button to select high pressure. Set the timer for 90 minutes. The Instant Pot will preheat and automatically start the cooking time.
Once the 90 minutes is up, wait 15 minutes and carefully vent the lid and open. Remove the brisket from the pot. Cover and set aside while keeping warm. Add remaining ingredients to the Instant Pot. Select the "Manual" button followed by the "Adjust" button to select high pressure. Set the timer for 15 minutes. The Instant Pot will again pre-heat and being cooking.
After the 15 minutes, carefully vent and remove the lid. Serve along side sliced brisket. Enjoy!