French Onion Chicken Soup

Name: French Onion Chicken Soup
Ingredients: 3 pounds yellow onions (4 large or 6 medium); 6 tablespoons unsalted butter, divided; 1 teaspoon dried thyme; 2 dried bay leaves; 2 1/2 teaspoons kosher salt, divided, plus more as needed; 1/2 teaspoon freshly ground black pepper; 3 medium carrots; 3 medium stalks celery; 3 cloves garlic; 2 pounds boneless, skinless chicken breasts or thighs; 3/4 cup dry white wine; 1 tablespoon dry sherry; 1/4 cup all-purpose flour; 6 cups low-sodium chicken broth; 1/2 baguette; 2 ounces Gruyère cheese (about heaping 1/2 cup)