Generously season both sides of the chuck roast with ground black pepper.

Set the Instant Pot to 'Sauté' mode. Once hot, add olive oil to the pot. Sear the seasoned chuck roast for about 3 minutes per side until golden brown.

Add the diced yellow onion to the still-hot pot and sauté for about 2 minutes, stirring occasionally, until the onion is tender. Pour in the beef broth and use a wooden spoon to scrape up any browned bits from the bottom of the pot.

Return the seared chuck roast to the Instant Pot, placing it on top of the softened onions and broth. Sprinkle the ranch dressing powder and au jus gravy powder evenly over the roast. Add the unsalted butter in several pieces over the top. Arrange the pepperoncini peppers on and around the roast, then pour the pepperoncini brine over everything.

Secure the Instant Pot lid and ensure the pressure release valve is set to 'Sealing.' Press the 'Pressure Cook/Manual' button and set the timer to High Pressure for 60 minutes (for a 3 lb roast) or 70 minutes (for a 4 lb roast).

Allow the Instant Pot to naturally release pressure for 10-15 minutes after cooking; wait until the pressure pin drops before carefully opening the lid. Remove the beef and set aside. In a small bowl, mix cornstarch and water to make a slurry. Set the Instant Pot to 'Sauté' and whisk in the slurry, letting it simmer for 1-2 minutes until the gravy thickens.

Shred the cooked beef using two forks, then drizzle it with the luscious gravy. Serve immediately with your favorite side dishes.