1. Place onions in a 3 1/2- to 4-quart slow cooker; top with pork. Combine wine, sugar, 1 ½ teaspoons each vinegar and Worcestershire sauce in small bowl; pour over pork. Cover and cook on High for 4 to 5 hours (Low: 8 to 10 hours) until falling apart.
2. Meanwhile, combine mayonnaise with remaining ½ teaspoon each balsamic vinegar and Worcestershire sauce; cover and refrigerate until needed.
3. Slice baguettes in half horizontally and toast under the broiler.
4. Remove meat from slow cooker. Strain onions from juices left in crock; discard juices.
5. Use tongs or 2 forks to pull pork; mix in strained onions. Serve on French baguettes with seasoned mayonnaise to taste, lettuce and tomatoes. Refrigerate or freeze any leftover meat for later use.