Pull the chuck roast out of the refrigerator 20 to 30 minutes before cooking so the center is not ice cold when it hits the slow cooker. Pat it dry all over with paper towels.
Set the roast in the bottom of a 6-quart slow cooker. If one side has a thick fat cap, put that side up so it can baste the meat as it cooks.
Pour the 1/4 cup of pepperoncini brine around the roast. Scatter the ranch seasoning evenly across the top, then do the same with the au jus gravy mix.
Set the stick of butter right in the center of the roast. Arrange the pepperoncini peppers on top, along the edges and over the butter, so they flavour the meat as it cooks.
Put the lid on and cook on LOW for 8 hours, or on HIGH for 4 to 5 hours. Leave the lid alone while it cooks.
You will know it is done when the meat shreds with gentle pressure from a fork. If it fights back, it is not ruined and it is not tough. It is just not finished yet. Give it another 30 to 45 minutes and test again.
Move the roast to a cutting board or platter and shred it into large chunks with two forks, pulling out and discarding any big pieces of fat. Skim fat off the surface of the cooking liquid if you want a cleaner finish.
Return the shredded beef to the slow cooker and toss it through the juices. Let it sit on warm for 10 to 15 minutes so the meat soaks the flavor back in, then taste and adjust the salt.
Serve hot over mashed potatoes, egg noodles, or rice, with plenty of the juices spooned over the top and a pepper or two on each plate.
