Optional step: In a large skillet on medium-high heat, sear the roast for 5 minutes on each side.
To a slow cooker, add the roast, onion if using, and pepperoncini juice. Sprinkle roast with dressing mix, gravy mix, and peppers. Top with butter slices.
Cook 8-10 hours on low or 4-6 hours on high, or until roast is fork-tender. If the roast is not fork tender, cook for another 30 minutes and check again.
Remove the roast from the slow cooker.
For optional gravy, mix cornstarch and water to create a slurry. Stir slurry into the juices in the slow cooker, cover, and turn to high. Cook for 5-10 minutes, or until slightly thickened.
Shred meat with a fork, or slice against the grain. Mix in with the juices if desired, or serve with the gravy. Serve with peppers and juices or gravy along with mashed potatoes, egg noodles, or rice.
Notes
Prep everything ahead of time and refrigerate until ready for cooking.
Refrigerate leftovers in an airtight container for up to 4 days, they are perfect for sandwiches!
Freeze portions in zippered bags for 3 months.
Time calculated with slow cooker set on the high setting.