Begin by peeling, halving, and thinly slicing 3 to 4 yellow onions. In a Dutch oven, melt 4 tablespoons of unsalted butter over medium heat. Add sliced onions, 1 teaspoon of dried thyme, 2 dried bay leaves, kosher salt, and black pepper. Cook for 35 to 45 minutes until golden brown.

Chop 2 medium carrots and 2 stalks of celery into small pieces. Mince 3 cloves of garlic. Set aside.

Once the onions are golden, add another tablespoon of unsalted butter. Stir in the chopped carrots and celery, cooking for about 5 minutes until softened.

Season the chicken with salt and place in the Dutch oven. Pour in 1 cup of dry white wine and 1/4 cup of dry sherry, allowing to reduce for 2-3 minutes. Sprinkle in 1/4 cup of all-purpose flour and stir for 1 minute. Gradually stir in 6 cups of low-sodium chicken broth and add chicken back. Simmer for 10-20 minutes until cooked through.

Slice baguette into 1-inch thick slices. Grate 1 cup of Gruyère cheese and sprinkle over baguette slices. Broil for 30 seconds to 1 minute until cheese is bubbly and golden.

Once chicken is cooked, remove and shred using two forks. Return shredded chicken to pot and remove bay leaves. Stir and adjust seasoning with salt and pepper to taste.

Ladle soup into warm bowls and top each serving with cheesy toast. Serve warm.