French Onion Soup

Name: French Onion Soup
Ingredients: 2 1/2 pounds large yellow onions (3 to 4); 4 tablespoons (1/2 stick) unsalted butter; 5 sprigs fresh thyme; 2 fresh or dried bay leaves; 1 1/2 teaspoons kosher salt, plus more as needed; 1/4 teaspoon freshly ground black pepper, plus more as needed; 1 tablespoon all-purpose flour; 1 cup dry white wine; 6 cups low-sodium beef broth; 2 tablespoons brandy or Cognac (optional, but highly recommended); 8 ounces Gruyère cheese; 1 (about 9-inch-long) piece baguette (about 6 ounces)