In a large pot or Dutch oven, melt the butter and olive oil over medium heat. Add the sliced onions and cook, stirring frequently, until they are caramelized and golden brown, about 30-40 minutes.

Add Garlic and Wine

Stir in the minced garlic and cook for another 2 minutes. Add the dry white wine and scrape the bottom of the pot to deglaze. Allow the wine to reduce by half.

Combine Stocks

Add the beef stock, chicken stock, bay leaf, and thyme to the pot. Season with salt and black pepper. Bring to a boil, then reduce heat and let simmer for 30 minutes.

Prepare the Croutons

While the soup is simmering, preheat the oven to 375°F (190°C). Toss the cubed baguette with olive oil, salt, and pepper. Spread on a baking sheet and bake for 10-12 minutes or until golden and crisp.

Serve the Soup

Remove the bay leaf from the soup. Ladle the soup into bowls, top with grated Gruyère cheese, and add the croutons. Garnish with fresh parsley before serving.