1. Heat oven to 350 degrees.
2. In skilled coated with cooking spray over medium heat, cook onions 10 minutes or until lightly browned.
3. Place rolls on work surface; fill with roast beef and mozzarella cheese.
4. Bake 20 minutes or until cheese is melted.
5. Remove from oven; top each with 1/2 cup onions and 2 tablespoons blue cheese dressing.
6. Place remaining bread top on sandwich.