Caramelize the onions

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

Deglaze and simmer

• Sprinkle the flour over the onions and stir for a minute.
• Pour in the white wine and scrape up the browned bits.
• Add the beef stock, thyme and bay leaf, then season with salt and pepper.
• Simmer gently, partially covered, skimming any foam.

Toast the bread

• Arrange the baguette slices on a baking tray.
• Toast in the oven at 200 °C until crisp and lightly golden.

Broil with cheese

• Ladle the soup into ovenproof bowls and remove the thyme and bay leaf.
• Top each bowl with toasted baguette slices and a generous layer of grated Gruyère.
• Broil until the cheese is melted and bubbling and golden.