Cut the roast into 3-4 chunks, trimming and discarding excess fat. In a small bowl, combine the salt, pepper, garlic powder and onion powder. Mix well and rub over all surfaces of beef chunks.
Set Instant Pot to saute. Heat the olive oil and add the pieces of beef to the pot. Cook for 2 minutes/side. Do this in batches, if needed, and transfer to a cutting board.
Add the onion and saute 3 minutes, or until fragrant. Add beef stock/broth and deglaze pot (scrape bits from the bottom of the pan with a spoon). Add the seared meat to pot, along with pepperoncinis, pepperoncini juice, both dry mixes and the butter. Cover and cook on high pressure (pressure cook setting) for 45 minutes. It will take the pot approximately 17 minutes to build up pressure before it begins cooking. Let the pressure release naturally for 10 minutes, then do a manual release.
Remove the beef from the pot and place on a cutting board. Shred meat using a fork.
Combine the water and cornstarch in a small bowl and mix. Set the pot to saute and bring the liquid to a boil. Whisk in the water-cornstarch mixture and let simmer until thick.
Add shredded meat back to the pot. Let sit 5 minutes. Serve with mashed potatoes or rice!