Heat the Instant Pot on Saute mode (high setting). Add the oil.

Season the London broil with salt and pepper. Set it into the Instant Pot and saute for 3 minutes per side. When done, remove the meat from the pot.

Pour in the beef stock and deglaze the bottom of the pot, removing all the browned bits.

Add the Worcestershire sauce, balsamic, tomato paste, and thyme. Whisk to combine.

Add the meat back to the pot.

Secure the lid and set the vent to sealing (if necessary for your model).

Set to cook on high pressure for 10 minutes. When the time is up, allow a 10 minute natural pressure release, then carefully release any remaining pressure. Remove the lid. This timing leads to a well-done fall apart meat, like a roast. If you want a medium rare to medium meat, cook it for 8 minutes on high pressure with a quick release.

Transfer the London broil to a plate to rest.

If you want to thicken the juices for gravy, turn on saute mode. Mix 2 tablespoons cornstarch with 2 tablespoons water.

When the liquid is boiling, pour in the cornstarch slurry. Whisk and cook until thickened.

Slice the meat and serve with the gravy.