The butter is completely optional. You can leave it out and save some calories, but it enriches the gravy.
I don’t recommend leaving out the peppers since they add flavor and the vinegar from the brine helps to tenderize and balance the richness. They're not spicy at all once they’ve cooked for several hours – all that’s left is a little “zip” or “zest.”
If you’re concerned about spice, try it with just the pepperoncini juice.
If you don’t want to use a slow cooker, this roast can just as easily be made in the oven. Place the ingredients in large oven-safe baking dish or roasting pan and cover tightly with foil. Bake at 300°F for about 1 hour per pound.