After 40 minutes, vent Instant Pot and test roast for doneness. It should be fork tender but not falling apart. If necessary, pressure cook on high for an additional 10-20 minutes.
Remove beef and set aside. Place potatoes on the bottom of the Instant Pot, making sure they are submerged ⅔ of the way (add additional beef stock if needed). Place carrots on top of potatoes. Place roast on top of carrots and finally cover with onions.
Close Instant Pot and set on Pressure Cook - High. Cook for an additional 20 minutes.
Once it is finished vent using the Quick Release button and remove roast, carrots and onions.