Cook mushrooms and onion in large nonstick skillet sprayed with nonstick cooking spray over medium heat, stirring occasionally, 7 minutes or until mushrooms are tender and liquid is evaporated. Stir in Wish-Bone® Chunky Blue Cheese Dressing and cheese just until combined. Cut bread in half to make 2 sandwiches; open each sandwich. Top each with 1/2 of the turkey and 1/2 of the cheese mixture. Broil until cheese is melted.