In a large skillet, saute onions in butter until crisp-tender. Transfer to a 5-qt. slow cooker. Add the broth, Worcestershire sauce and bay leaves., Cover and cook on low for 5-7 hours or until the onions are tender. Discard bay leaves., Ladle soup into ovenproof bowls. Top each with a slice of toast; sprinkle with desired amount of cheese. Place bowls on a baking sheet. Broil for 2-3 minutes or until cheese is lightly golden.