Turn on the Instant Pot and set it to "sauté" mode (30 minutes should give you enough time to brown the meat and cook the onions). Put the oil into the inner pot of the Instant Pot. Next, add pepper to each side of the chuck roast. When the instant pot reads "hot" on the display screen, place the chuck roast into the inner pot. Cook on each side for about 3 minutes until golden brown. (Skip this step if you prefer not to brown your meat. But be sure to add 2-3 extra minutes to the pressure cook time.)
Remove the roast from the Instant Pot. Add onion; cook and stir about 2 minutes, or until onion is tender. Next, add in beef broth and deglaze by scraping the bottom of the pot. Then, layer the ingredients in by adding the chuck roast; followed by ranch dressing mix powder, au jus gravy mix, butter, and pepperoncini pepper and its juice.
Place the lid on and make sure the knob is set to "sealing". Press "manual" (older model) or "pressure cook" (newer model) and set it to cook on High Pressure for 60-70 minutes. (60 minutes for 3 pounds of chuck roast. Add an extra 10 minutes of cook time for every pound. For example, 3 lbs = 60 minutes, 4 lbs = 70 minutes). Once it is done cooking, let the pressure naturally release until the pin drops down and the lid can be opened.