Turn Instant Pot to Saute setting. When hot, add oil and roast. Brown on all sides.
Add onions and garlic when roast is almost finished browning. Stir and brown onions 1-2 minutes.
Add broth, au jus, and seasonings. Stir. Seal lid into place and set to pressure cook for 80 minutes. When time is up, allow to natural release for 15 minutes, then quick release remaining pressure.
Use tongs to remove roast to cutting board. Slice and return to juices.
To Make Gravy:
When the roast is out of the pot, set to Sauté again. Add 2 tablespoons of cornstarch (or flour) to ¼ cup cold water. Stir until smooth. Pour into pot juices, and stir, until gravy is thickened.