Season the beef with salt and pepper on all sides.
Heat the olive oil in the Instant Pot set on the sauté setting. Sear the beef on all sides until nicely caramelized. Set aside.
Add the onion and garlic to the Instant Pot and cook until just fragrant, about 2 minutes. Turn off the Instant Pot.
Add the thyme and the beef stock to the Instant Pot, stirring with a wooden spoon to bring up the fond. Add the trivet to the Instant Pot, then add the beef.
Cook on high for 3 minutes, then reduce to the warm setting. Keep the pot on warm for 3 hours. Take the internal temperature of the beef: when it's at 135 degrees Fahrenheit, it will be perfectly medium-rare.
Remove the beef from the Instant Pot and strain out the juices. Return to the Instant Pot and cook on the sauté setting to reduce to desired thickness.