Optional: sear the roast in a hot, lightly oiled skillet for 2–3 minutes per side, until deeply browned.
Place the chuck roast in the slow cooker and sprinkle the ranch seasoning mix and the au jus gravy mix evenly over the top.
Lay the butter on top of the roast, arrange the pepperoncini peppers around it, pour the pepperoncini juice over everything, and scatter the garlic on top if using.
Cover and cook on LOW for 8–10 hours or on HIGH for 5–6 hours, without lifting the lid, until the beef is fall-apart tender and shreds easily with a fork.
Shred the beef directly in the slow cooker with two forks, pulling out and discarding any large pieces of fat.
Let the shredded beef rest in its juices for 10–15 minutes so it soaks the flavor back up.
Sprinkle with chopped fresh parsley and serve hot over mashed potatoes, buttered noodles, rice, or piled into sandwiches.