Put the onion, thyme, garlic, pickling spice, brisket and 6 cups water into an Instant Pot®. Follow the manufacturer's guide for locking the lid and preparing to cook.
Seal and cook on high pressure for 85 minutes. Follow the manufacturer's guide for quick release, then remove the meat and set aside. Strain the liquid through a fine mesh strainer into a bowl and return 2 cups back to the Instant Pot® along with the butter.
Add the rutabaga, leeks, carrots and cabbage to the Instant Pot®. Seal and cook on high pressure for 4 minutes. Quick release the steam. Thinly slice the brisket across the grain and transfer to a serving platter. Arrange the vegetables around the meat, sprinkle with the parsley and serve with the sour cream, horseradish and mustard on the side.
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