Slice four large yellow onions into thin rings. Melt four tablespoons of unsalted butter in a heavy-bottomed pot over medium heat. Add onions and a pinch of kosher salt, cooking for 35-45 minutes until caramelized.

Once onions are caramelized, add one teaspoon of dried thyme, two dried bay leaves, and black pepper to taste. Stir well and cook for 2-3 minutes.

Dice two carrots and one celery stalk, adding them to the pot along with three minced garlic cloves. Stir and cook for 5 minutes until softened.

Pour in half a cup of dry white wine and half a cup of dry sherry, stirring to deglaze the pot. Allow to simmer for 2-3 minutes. Sprinkle two tablespoons of all-purpose flour, stirring until thickened.

Gradually add six cups of low-sodium chicken broth while maintaining a gentle simmer. Add two boneless, skinless chicken breasts or thighs and cover. Simmer for 12-20 minutes.

Remove the cooked chicken and shred using two forks. Return the shredded chicken to the pot and stir to combine.

Preheat your broiler. Slice a baguette into thick pieces, place on a baking sheet, top with grated Gruyère cheese, and broil for 2-3 minutes until melted.

Ladle the soup into bowls and top with a cheesy toast. Serve hot and enjoy!