Peel and thinly slice the yellow onions. In a large pot, melt 4 tablespoons of unsalted butter over medium heat. Add onions, thyme, bay leaves, kosher salt, and black pepper. Cook for 35–45 minutes, stirring frequently, until onions are caramelized and golden brown.

While the onions caramelize, wash and chop 2 carrots and 2 stalks of celery. Mince 4 cloves of garlic and combine in a bowl.

Add 1 tablespoon of butter to the pot with caramelized onions. Toss in carrots, celery, and garlic, sautéing for 5 minutes until softened.

Add 1-1.5 pounds of chicken to the pot, seasoning with salt and pepper. Sauté for 5 minutes until browned. Pour in dry white wine and dry sherry, then simmer for 2–3 minutes.

Sprinkle 2 tablespoons of flour over the mixture, stir well, and gradually add 6 cups of broth while stirring. Bring to a simmer and cook for 10–20 minutes.

Remove chicken from the pot, shred it into bite-sized pieces, returning to the pot and discarding bay leaves.

Slice a baguette and top each piece with Gruyère cheese. Broil for 30 seconds to 1 minute until golden.

Ladle the soup into bowls, top with cheesy toasts, and serve hot.