French Onion Soup Gratinee

Ingredients

• 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
• 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
• salt
• freshly ground black pepper