In a large Dutch oven, melt 4 tablespoons of unsalted butter over medium heat. Add 4 sliced yellow onions, 1 teaspoon of dried thyme, 2 bay leaves, 1 teaspoon of kosher salt, and 1/2 teaspoon of black pepper. Sauté for 35–45 minutes, stirring occasionally, until the onions are soft, deep amber, and fragrant.

Stir in 2 diced carrots and 2 diced celery stalks into the caramelized onions. Increase the heat slightly and cook for about 5 minutes, stirring often.

Pour in 1/2 cup of dry white wine and 1/4 cup of dry sherry, scraping up any browned bits from the bottom of the pot. Allow it to simmer for about 2–3 minutes.

Sprinkle in 1/4 cup of all-purpose flour, stirring continuously for about 1 minute.

Gradually pour in 6 cups of low-sodium chicken broth while stirring. Then, add 1 pound of boneless, skinless chicken. Bring to a gentle simmer and cook for 10–20 minutes, or until the chicken is cooked through.

Slice a baguette into 1-inch pieces and top each slice with a generous amount of grated Gruyère cheese. Broil for 30 seconds to 1 minute until bubbly.

Remove the chicken from the soup and shred it. Return the shredded chicken to the pot and stir to combine. Taste and season with additional salt and pepper if needed.