Rub the Montreal seasoning all over the roast along with a bit of coarse salt and pepper.
Warm a bit of olive oil in a cast iron skillet over medium-high heat.
Add the roast and sear until golden brown on both sides, about 2 minutes per side. Transfer the roast to a greased 6 quart crockpot.
Add butter to the skillet and once melted, add the onion, carrots, garlic, 8 pepperoncini and ¼ cup pepperoncini juice, all 3 seasoning packets, and the beef stock. Stir to combine and let simmer for about 5 minutes.
Pour the sauce mixture over the roast in the crockpot. Cover with the lid and cook on low for 8 hours until roast is fork-tender. (Do not open the lid while the roast cooks.)
Remove roast from the crockpot; cut away any fatty pieces, then shred with 2 forks.
Serve over mashed potatoes with some of the gravy. This is also delish in Hoagie rolls.
