Place the metal trivet inside of the Instant Pot. Add the sliced onion, minced garlic, pickling spices, and water to the Instant Pot. Place the corned beef brisket, fat side up, on the rack on top of the onions. Place the lid on the Instant Pot and lock it into place. Set the steam release knob to the sealing position. Press the Manual button and set it to 85 minutes (1 hour 25 minutes). Corned beef brisket should be cooked until the internal temperature of 145°.
When the cooking cycle has finished, let it naturally release for 20 minutes, then manually release the remaining pressure. Once all steam has been released and the pressure valve has lowered, remove the lid. Remove the corned beef from the pressure cooker and place it on a cutting board and cover with aluminum foil to keep warm.
Strain the cooking liquid into a small bowl to remove the cooked onions and spices. Return 1 1/2 cups of the strained cooking liquid to the Instant Pot and save the remaining liquid.
Add the potatoes, carrots, and cabbage to the pressure cooker. Place lid on the Instant Pot and lock it into place. Set the steam release knob to the sealing position. Press the Manual button and set it to 4 minutes. After the cooking cycle has ended, manually release the pressure. Once all the steam has been released and the pressure valve has lowered, remove the lid.
Slice corned beef against the grain. Spoon reserved cooking liquid over the corned beef slices. Remove vegetables from the Instant Pot with a slotted spoon and serve with the corned beef.
Store leftovers in an airtight container and refrigerate. Reheat leftovers in the microwave.
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Notes
Check the label of the corned beef brisket to make sure it is gluten-free.
Corned beef brisket should be cooked until the internal temperature of 145°.
Store leftovers in an airtight container and refrigerate. Reheat leftovers in the microwave.