Caramelize the onions

• Melt the butter with the olive oil in a stockpot over medium heat.
• Add the sliced onions with a pinch of salt and cook slowly, stirring often, until deep golden brown.

Build the soup base

• Stir in the garlic and cook until fragrant.
• Sprinkle over the flour and cook, stirring, for a minute.
• Pour in the white wine and let it reduce.

Simmer the soup

• Add the beef stock, thyme and bay leaf.
• Season with salt and pepper and simmer gently, partially covered.

Top and gratinate

• Ladle the soup into ovenproof bowls or a baking dish.
• Top with toasted baguette slices and a generous layer of grated gruyère.
• Bake until the cheese is melted and bubbling.

Serve

• Remove the bay leaf and thyme sprigs and serve the soup hot.