To Make the Peppercorn Dressing: In a small bowl whisk together all the ingredients. Store the dressing in refrigerator until ready to use.
To Prepare the Steak: Preheat broiler and line a baking pan with foil. Place the steak on the pan and sprinkle with Worcestershire sauce. Sprinkle with a little garlic powder, seasoned salt and pepper. Broil for about 4-5 minutes. Remove from oven, turn over and sprinkle with Worcestershire sauce, garlic powder, seasoning salt and pepper. Broil for another 4-5 minutes until desired doneness. Remove from oven, reserve any juices from the meat, cool slightly and slice into thin strips on the diagonal.
To Make the Cheese Steak Mixture: In a large pan, cook together the charbroiled steak strips, reserved juices from the meat, red peppers and onions until the peppers and onions are soft. Cover and keep warm.
In a large bowl toss together the spring mix lettuce and 4 tablespoons of peppercorn dressing. Add more dressing, if desired.
Place one toasted bun opened-faced, on each plate. Brush each with balsamic vinegar.
Spoon the steak and vegetables mixture on top of each bun. Top each with one tablespoon mozzarella cheese.
Arrange the tossed lettuce on top of the cheese, completely covering the bun. Sprinkle each with 1 tablespoon Parmesan cheese.
Serve at once.