Melt 2 tablespoons of the butter and the canola oil in a heavy pot over high heat. Sear both sides of the chuck roast until very browned, about 5 minutes in all. Pour in the beef broth and 1 cup water. Add the rosemary, and then pour in the pepperoncini with their juices. Now cover the pot and simmer until the meat is tender and falling apart, 4 to 5 hours.
Meanwhile, heat a skillet over medium heat and add the remaining 1 tablespoon butter. Add the onions and saute until light golden brown. Set aside.
Remove the roast from the pot and shred the meat completely using two forks. Return the meat to the cooking liquid and keep warm.
To serve, slice wedges out of the tops of the deli rolls. Heap a generous portion of meat on each roll, and then spoon some of the cooking liquid over the meat. Top with a few peppers from the pot and plenty of caramelized onions. Top the sandwiches with the wedges of roll and serve to a roomful of ravenous guests. You¿ll win friends and influence people.
For a variation, lay thinly sliced cheese on top of the meat before adding the peppers and onions. Serve with dishes of the cooking liquid on the side for dipping if desired.