Preheat oven to 350 degrees. Chop onions to chewable size. Cut 4 slices of bread into small crouton size then bake until golden. Set aside. Saute onions in butter on low heat until soft and dark brown. Salt and pepper to taste. In a saucepan, heat both cans of beef broth plus 1 can of water and cook to slight boil. Add Italian bread crumbs and onions. Pour broth with onions into soup bowls. Add Parmesan cheese and croutons to each bowl, then layer each bowl with a slice of mozzarella cheese. Place into oven. Bake until cheese is crisp and brown. Let sit for a few minutes before serving.