Cut butter into 4 pieces.
Trim off any large pieces of surface fat from 1 (3-pound) boneless beef chuck roast. Pat dry with paper towels.
Heat 1 tablespoon vegetable or canola oil in a 6-quart or larger Instant Pot or electric pressure cooker on the highest Sauté setting until shimmering. Add the roast and sear on all sides until browned, about 10 minutes total.
Transfer to a plate.
Pour 1/4 cup water into the pressure cooker and scrape up any browned bits loose.
Return the roast and any accumulated juices to the pressure cooker. Sprinkle the roast with 1 packet ranch mix and 1 or 2 packets au jus gravy mix. Top with the butter. Add 2 cu-3 TB brine from pepperoncini peppers over the roast. Add pepperoncini peppers.
Lock the lid on and make sure the pressure valve is set to seal. Set to cook under HIGH pressure for 55 minutes. It will take about 15 minutes to come up to pressure.
When the cook time is up, let the pressure naturally release for 15 minutes. Quick release any remaining pressure. Shred the roast into bite-sized pieces with tongs.
Serve with potatoes, rice, tortillas (this makes a great street taco filling too, just add fixings!), or whatever you like. If you have leftovers, great. (I seldom, if ever, do!)