Caramelize the onions

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

Build the soup

• Pour in the white wine and scrape up the browned bits.
• Add the beef stock, thyme and bay leaf, then simmer to let the flavours meld.
• Season with salt and pepper and discard the bay leaf.

Toast the croutons

• Arrange the baguette rounds on a baking tray.
• Toast them until crisp and golden.

Gratinate the soup

• Ladle the soup into ovenproof ramekins.
• Top each with baguette rounds and a generous layer of Gruyère.
• Broil until the cheese is melted and bubbling.

Sear the steak

• Season the sliced steak with salt and pepper.
• Sear it in a hot frying pan with a little olive oil until browned but still juicy.
• Set aside and keep warm.

Assemble the grilled cheese

• Butter the sourdough slices on the outside.
• Layer cheddar, mozzarella and the seared steak between two slices.
• Grill in the pan, pressing lightly, until the bread is crisp and the cheese has melted.
• Repeat with the remaining bread, cheese and steak.

Serve

• Cut the grilled cheese in half and serve hot alongside the French onion soup.