Heat oven to 350°F. Rub flank steak on both sides with olive oil. Sprinkle the mushrooms and olives over the steak, if using. Place a layer of prosciutto, then a layer of mortadella, mozzarella, and then pepperoni. Season with some of the Italian seasoning. Roll steak up around the filling and tie with butcher's string. Rub the outside of the roll with remaining Italian seasoning, garlic powder and steak seasoning. Heat a large cast iron skillet over high heat. Sear roll over the entire outside. Then leave roll in the skillet, and place skillet in oven. Roast for 1 hour till internal temperature reaches 160°F in the center. Let rest for 10 minutes before slicing and serving.