Set the Instant Pot to the saute function, and add in the olive oil.
Cut the chuck roast in half, and season it well with salt and pepper to taste on each side.
Place the beef into the heated Instant Pot, and sear for 4 minutes on each side.
Remove the beef from the Instant Pot, and add in the onion. Saute the onion until it begins to soften, and then turn the Instant Pot off.
Add the garlic to the Instant Pot, and then mix in the worcestershire sauce, broth and tomato paste. Stir to combine well.
Return the beef to the Instant Pot, and place the sprigs of fresh thyme in with it.

Close the lid on the Instant Pot, and make sure that the release valve is set to sealing.
Set the Instant Pot to cook on manual for 60 minutes. Allow the Instant Pot to do a slow release for 10 minutes after the cook time has ended, and then move the release valve to the venting position to relieve any additional pressure.
Remove the lid, and place the potatoes and carrots on top of the beef. Close the lid again, making sure the release valve is set back to sealing, and set the Instant Pot to manual for 10 minutes. Again, allow the Instant Pot to naturally release for 10 minutes after the cook time is up before releasing the additional pressure by moving the release valve to the venting position.
Take the lid off of the Instant Pot again, and remove the thyme sprigs and beef. Discard the thyme sprigs, and shred the beef before returning it to the pot.
If you wish to thicken the liquid in the pot roast, mix the cornstarch with a small amount of water, and pour it into the pot. Stir to combine, and allow it to thicken.