Heat oil in the pressure cooker over medium-high heat with the lid open. Brown the roast on all sides in the hot oil. In a small C. or bowl, mix together the Italian salad dressing mix, Ranch dressing mix, and gravy mix. Sprinkle them evenly over the roast. Pour in the beef broth and add the chopped onion. Seal and lock pressure cooker, and cook over high heat to build pressure until the indicator sounds (mine whistles). Turn heat down to medium, and cook for 45 minutes.
Remove from heat and let stand for 5 minutes. Run under cold water to help release the pressure before unsealing the lid. You can use the juices as an au jus, or thicken with flour or cornstarch to make wonderful gravy.