Caramelize the Onions:

• Heat the butter and oil: In a large pot or Dutch oven, melt the butter with olive oil over medium heat.
• Cook the onions: Add the sliced onions and a pinch of salt. Cook for 25-30 minutes, stirring frequently, until the onions are golden brown and caramelized. Reduce the heat if it starts to burn.

Build the Soup:

• Add the garlic and flour: Stir in the minced garlic and cook for 1 minute. Sprinkle in the flour, stirring constantly, and cook for another minute to remove the raw taste.
• Deglaze with wine: Pour in the white wine, scraping up any browned bits from the bottom of the pot. Simmer until the wine reduces by half.
• Add the stock and herbs: Stir in the beef stock, bay leaf, and thyme. Bring to a gentle boil, then reduce the heat and simmer for 15-20 minutes. Adjust seasoning with salt and pepper.

Prepare the Topping:

• Toast the bread: Lightly toast the baguette slices under the broiler or in a toaster until golden brown.
• Top with cheese: Sprinkle the Gruyère cheese over the bread slices and broil until melted and bubbly.

Serve the Soup:

• Remove the bay leaf from the soup.
• Ladle the soup into oven-safe bowls, place a cheesy bread slice on top, and broil for 2-3 minutes until golden. Alternatively, serve with the cheesy bread on the side.