Flank Steak, Caramelized Onion, Mushroom and Blue Cheese Panini

I was in the mood for a panini sandwich for dinner. I decided to marinate some flank steak in a balsamic, Dijon and garlic marinade. Looking through my refrigerator and pantry I found an onion, mushrooms, mayonnaise, garlic and low fat blue cheese (it's really good and has great flavor) - all the ingredients you need for a tasty steak sandwich. I always make my panini sandwiches "dry" instead of