Season roast: Sprinkle roast all over with salt and black pepper.
Sear roast: Heat olive oil in a 12-inch cast-iron skillet over high until shimmering. Add roast; cook, turning once halfway through cooking time, until deeply browned on both sides, about 10 minutes. Transfer roast with drippings to a 6-quart slow cooker.
Add ingredients to slow cooker: Dot roast all over with butter. Sprinkle evenly with ranch dressing mix and onion soup mix. Place pepperoncini peppers over top and around roast.
Cook roast: Cover and cook until very tender, about 5 hours on HIGH or 7 hours on LOW.
Shred meat: Using 2 forks, shred roast in slow cooker into bite-size pieces.
Serve: Using a slotted spoon, serve roast over mashed potatoes. Garnish with cooked pepperoncini peppers and parsley.
