In a large Dutch oven, melt 4 tablespoons of unsalted butter over medium heat. Add the sliced yellow onions, dried thyme, 1 teaspoon of kosher salt, and freshly ground black pepper. Cook for 35-45 minutes, stirring frequently until the onions turn deep amber brown and are beautifully caramelized.

While the onions are caramelizing, wash and slice the carrots and celery into small pieces. Mince the garlic cloves and combine all the vegetables in a bowl.

Once the onions are caramelized, add the remaining 2 tablespoons of butter to the pot. Toss in the prepared carrot, celery, and garlic mixture. Sauté for about 5 minutes, allowing the vegetables to soften.

Pour in 3/4 cup of dry white wine and 1 tablespoon of dry sherry, stirring well to deglaze and lift the flavorful bits stuck to the bottom. Cook for 2-3 minutes, letting the alcohol evaporate. Sprinkle 1/4 cup of all-purpose flour over the mixture and stir for 1 minute to combine.

Slowly pour in 6 cups of low-sodium chicken broth while stirring. Add the 2 pounds of chicken breasts or thighs to the pot. Bring to a gentle simmer and cook for 10-20 minutes, or until the chicken is cooked through.

Once the chicken is fully cooked, carefully remove it from the pot. Let it cool slightly before shredding it into bite-sized pieces. Return the shredded chicken to the pot, stir to combine, and taste for seasoning.

Preheat your broiler. Slice the baguette and place the pieces on a baking sheet. Generously top each slice with 2 ounces of grated Gruyère cheese. Broil until the cheese is melted and bubbly, about 2-3 minutes.

Ladle the warm French Onion Chicken Soup into bowls, top with the cheesy toast, and enjoy!