Turn the Instant Pot on “Sauté” mode. Add the oil. When hot, add the garlic, and onion to brown slightly. Then, add the beef to quickly sear on all sides.Hit “cancel” to turn off the “sauté” mode. Add the potatoes, carrots, turnips, liquid and herbs to the pot.Set to “Pressure Cook” mode on High for 20 minutes(if cut into chunks) or 60 minutes (if left as a whole roast). Press “Start”.Learn from my mistakes and let the pressure release naturally. (I made the mistake of pushing for quick release and ended up with a starchy mess all over my kitchen).When the pressure has been released, add in the mushrooms. At this point, your meat should be completely cooked. This is also a good time for a taste test to see if you need any more seasoning.Set to “Pressure Cook” mode on Medium for 5 minutes. Press “Start”. When it is done, let the pressure release naturally.To make the gravy, remove all the meat and vegetables with a slotted spoon, then strain the remaining liquid through a mesh strainer.Return the liquid to the Instant Pot and set to “Sauté” mode on Medium heat. Whisk in the cornstarch, 1 tablespoon at a time. The liquid will start boiling and thickening. Whisk constantly for 2 minutes or until you reach a gravy consistency. Note that the liquid will also thicken as it cools down.Serve the meat and vegetables with the gravy.