Heat oil in uncovered pressure cooker on the "Brown" setting; heat with the lid open. Brown the roast on all sides in the hot oil.
In a small cup or bowl, mix together the Italian salad dressing mix, ranch dressing, and the gravy mix. Sprinkle them evenly over the roast. Pour in the beef broth and add the chopped onion.
Lock the cover into cooking position and follow directions for cooking. Cook at “High Pressure” for 55-60 minutes. When cooking has completed, use the natural release method to release pressure.
When pressure is completely released, remove the roast from the cooking vessel. You can use the remaining juices as an au jus or thicken with flour or cornstarch to make a gravy.