1. Combine mushrooms, onion, cheese and bacon in medium bowl; set aside.
2. Cover beef steak with plastic wrap; pound until steak is 1/4 inch thick.
3. Season steak on both sides with 1/2 teaspoon salt and 1/2 teaspoon pepper. Arrange mushroom mixture evenly over steak, leaving 1-inch border on all sides. Starting on long side, roll up steak to enclose mushroom mixture. Secure roll with kitchen twine.
Place tomatoes in slow cooker; stir in remaining 1/2 teaspoon salt, 1/2 teaspoon pepper and Italian seasoning. Add beef roll to sauce, turning one to coat. Cooked, covered, on HIGH 4 hours or on LOW 6 hours or until beef is tender.
5. Cut braciola diagonally into 1/2" thick slices. Serve over pasta or polenta topped with sauce.
Test Kitchen Tips For a larger flank steak, cut it in half horizontally. Pound the two halves, tope evenly with mushroom mixture and roll up each to make two smaller, more manageable rolls.