Set your instant pot to SAUTE mode.
Pat your pot roast completely dry. Season all sides with salt.
Add in the avocado oil, and brown your pot roast on both sides. About 7 minutes per side until crusty brown. Remove from the instant pot.
In a measuring cup, combine the beef broth + tomato paste. Stir to combine.
Pour in the beef broth to the instant pot, and scrape up any brown bits on the bottom. Add in the chuck roast, garlic cloves, rosemary + thyme.
Close with the lid, and make sure the vent is at SEALING. Hit CANCEL, then cook on HIGH PRESSURE for 60 minutes. Will take about 10-15 minutes to reach pressure, then it will start counting down from 60. After the 60 minutes is up, do a QUICK RELEASE, where you move the valve to VENTING to release all the steam. Once all the steam is gone and the float valve has dropped, you can safely open the instant pot.
To the instant pot, add in the onion, carrots and turnips on top of the pot roast. Close the lid, and cook on HIGH PRESSURE for 3 more minutes. Followed by a quick release.
Use a slotted spoon to transfer the vegetables to a serving platter. Use tongs to remove the pot roast to the same serving platter. Shred the pot roast with 2 forks into large chunks. Strain any solids from the cooking liquid, and reserve 2 cups to make gravy.