Melt 1 tablespoon of butter in a large frypan over medium heat. Add the onion; cook and stir until tender. Push onion to the side of the pan and add the mushrooms. Cook and stir until softened then add the capsicum and cook just until tender, about 3 minutes. Remove from the pan with a slotted spoon and set aside.
Add the remaining butter to the frypan. No need to clean the pan, just let it heat up a little bit. Place the steak in the frypan along with the pickled peppers. Season with salt and pepper. The steak cooks really fast, just a couple of minutes. Once the steak is mostly browned return the onion and capsicum to the pan. Cook until heated through.
Turn off the heat and place the slices of cheese over the top of the pile so they can melt. Scoop the whole pile into the awaiting bread, making sure to pour some of the juices onto that wonderful sandwich.