Place the top round and entire bottle of salad dressing in a resealable plastic bag and marinate in the refrigerator for 24 to 48 hours.
When meat is fully marinated and you're ready to prepare sandwiches, preheat oven to broil.
In a large skillet over medium heat, saute the onions and mushrooms in the butter or margarine for 10 minutes, or until onions are tender. Set aside.
Broil the meat for 5 to 7 minutes per side for medium rare. Remove from oven and slice thinly against the grain. Place slices evenly on the bread and cover with the onion mixture.
In a microwave-safe bowl, combine the cheese, salsa and olives. Microwave for 3 to 4 minutes on high, or until the cheese is melted. Stir well and serve with the sandwiches.