French Onion Soup Gratinee

4 tablespoons butter
1 teaspoon salt
2 large red onions, thinly sliced
2 large sweet onions, thinly sliced
1 (48 fluid ounce) can chicken broth
1 (14 ounce) can beef broth
1/2 cup red wine
1 tablespoon Worcestershire sauce
2 sprigs fresh parsley
1 sprig fresh thyme leaves
1 bay leaf
1 tablespoon balsamic vinegar
salt and freshly ground black pepper to taste
4 thick slices French or Italian bread
8 slices Gruyere or Swiss cheese slices, room temperature
1/2 cup shredded Asiago or mozzarella cheese, room temperature
4 pinches paprika

DIRECTIONS
Melt butter in a large pot over medium-high heat. Stir in salt, red onions and sweet onions. Cook 35 minutes, stirring frequently, until onions are caramelized and almost syrupy.
Mix chicken broth, beef broth, red wine and Worcestershire sauce into pot. Bundle the parsley, thyme, and bay leaf with twine and place in pot. Simmer over medium heat for 20 minutes, stirring occasionally. Remove and discard the herbs. Reduce the heat to low, mix in vinegar and season with salt and pepper. Cover and keep over low heat to stay hot while you prepare the bread. Preheat oven broiler. Arrange bread slices on a baking sheet and broil 3 minutes, turning once, until well toasted on both sides. Remove from heat; do not turn off broiler.
Arrange 4 large oven safe bowls or crocks on a rimmed baking sheet. Fill each bowl 2/3 full with hot soup. Top each bowl with 1 slice toasted bread, 2 slice Gruyere cheese and 1/4 of the Asiago or mozzarella cheese. Sprinkle a little bit of paprika over the top of each one. Broil 5 minutes, or until bubbly and golden brown. As it softens, the cheese will cascade over the sides of the crock and form a beautifully melted crusty seal. Serve immediately!

SUGGESTIONS
Just a couple of comments I thought might be helpful. The 1 teaspoon of salt in the beginning of the recipe cannot be omitted as it is necessary for the proper caramelization of the onions. However, at the conclusion of the simmering process, please be sure to taste the soup before deciding to add any more salt (or pepper). Also, the addition of balsamic vinegar at the end is very important--not only for taste--but also because there's a chemical in red onions that tends to turn the soup gray and cloudy, and the balsamic vinegar corrects that. Lastly, while my broiler does take 4-5 minutes to melt the cheese properly, that's probably because I place the baking sheet on a lower rack. Yours may take less time.

Make sure you cook the onions as suggested to almost a carmelized soft, syrupy, consistency. You may not need to adjust the salt in the end; the combination of beef and chicken broth and the Worcestershire and balsamic vinegar may be salty enough for many.

You can use croutons rather than sliced French bread. It is much easier than cutting through a thick piece of cheese-covered bread. Guests and family say they like that: it adds to the enjoyment and ease of eating without losing any visual appeal.