Creating this delightful soup is quite straightforward. Follow these simple steps to achieve a perfect bowl of White Onion Soup with Gruyère Croûtes:

1. Sauté Onions: In a large pot, melt the butter and olive oil over medium heat. Add the sliced onions and season with salt. Cook gently, stirring frequently, until the onions are soft and caramelized, about 25 minutes.

2. Add Flour: Once the onions are golden, sprinkle the flour over them and stir to combine. Cook for an additional 2 minutes to eliminate the raw flour taste.

3. Deglaze with Wine: If using, pour in the white wine and let it simmer for about 5 minutes until it reduces slightly.

4. Add Broth and Herbs: Pour in the broth, adding the thyme and bay leaf. Bring to a gentle boil, then reduce the heat and let it simmer for 20 minutes to allow the flavors to meld.

5. Season: Taste the soup and season with salt and pepper as needed. Remove the bay leaf before serving.

6. Prepare Croûtes: While the soup is simmering, preheat your oven to 350°F (175°C). Arrange the baguette slices on a baking sheet, sprinkle them with Gruyère cheese, and bake until the cheese is melted and bubbling, about 10 minutes.

7. Serve: Ladle the soup into warm bowls, top with the cheesy croûtes, and garnish with chopped parsley.

8. Enjoy: Serve hot, and savor the incredible flavors of your homemade White Onion Soup!