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 deeply golden and jammy.
• Add the garlic and cook until fragrant.

Deglaze and build the soup

• Sprinkle the flour over the onions and stir to coat.
• 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.
• Bring to a simmer and cook until slightly thickened.

Toast the bread

• Arrange the baguette slices on a baking tray.
• Toast in the oven until crisp and golden.

Broil with cheese

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