In 10-inch nonstick skillet, cook beef and onion over medium heat 8 to 10 minutes, stirring occasionally, until beef is thoroughly cooked; drain. Remove to large bowl; cool 5 minutes. Stir in rice, salt, pepper and egg.
Meanwhile, fill Dutch oven half full of water; heat to boiling. Remove 10 leaves from head of cabbage. Add leaves to boiling water; cook 8 minutes or until pliable. Drain; cool slightly.
Spray 5-quart slow cooker with cooking spray. To assemble, spoon beef mixture evenly onto center of cabbage leaves. Fold bottom third of leaf over filling; fold sides in toward center and roll up. In slow cooker, place rolls, seam sides down (some may overlap). In medium bowl, mix tomato sauce, brown sugar and Worcestershire sauce; pour over cabbage rolls.
Cover; cook on Low heat setting 5 hours.