1. Form ground sirloin into 6 equal-size patties and refrigerate.
2. Heat olive oil in a large nonstick skillet. Add onion and cook 5 minutes over medium
3. heat, stirring occasionally. Add pepper and mushrooms and cook an additional 4 to 5 minutes or until tender.
4. Heat gas grill to medium-high or coals in a charcoal grill to medium-hot. Lightly coat grill rack with oil or cooking spray
5. Grill burgers for 3 minutes per side, covered, or until internal temperature registers 160 degrees F on an instant-read thermometer. Turn off heat and place 2 slices of cheese on top of each burger. Cover and wait a few minutes until cheese melts.
6. Place burgers on Italian bread with onion and mushroom mixture spooned over top. Serve with pepperoncini, if desired.