Set Instant Pot to Saute, and heat oil.
Brown meat on both sides for 5-6 minutes.
Add half jar of Peperoncini peppers, and 1/4 cup of the juice.
Add the onion, beef broth, garlic, and seasonings.
Close lid, turn valve to sealing, and set to 70 minutes on Manual (High).
After cook time is finished, let natural release for about 15 minutes, then release the rest of pressure, and shred roast.
Serve warm with remaining sliced Peperoncini peppers as desired.