1. Pat the roast dry with paper towels; sprinkle with the salt and pepper.
2. Heat the oil in a large skillet over high; add the roast, and cook until browned, 2 to 3 minutes on each side. Place the roast in a 5- to 6-quart slow cooker; sprinkle the seasoning mix over the roast. Pour the pepperoncini peppers and liquid over the roast. Top with the butter, if desired.
3. Cover and cook on LOW until roast is very tender, about 8 hours, or on HIGH, about 5 hours.
4. Remove the roast from the slow cooker with a slotted spoon. Serve with roasted root vegetables. Garnish, if desired.