Begin by preparing the onions. Peel and thinly slice the 1.5kg (3.3 lb) of yellow onions. Aim for consistent thickness, about 0.3cm (1/8 inch) thick, as this will ensure even caramelization.

In a large, heavy-bottomed pot or Dutch oven, melt 115g (1/2 cup) of unsalted butter with 30ml (2 tbsp) of olive oil over medium-low heat. Once the butter is melted and shimmering, add the sliced onions.

Cook the onions slowly, stirring occasionally, for at least 45-60 minutes, or until they are deeply golden brown and caramelized. This slow cooking process is crucial for developing the rich flavor of the soup. Do not rush this step. If the onions start to brown too quickly, reduce the heat to low and stir more frequently. If they stick to the bottom, you can add a tablespoon of water to loosen them.

Once the onions are caramelized, add the 3 minced garlic cloves and cook for another 1-2 minutes until fragrant. Be careful not to burn the garlic.

Sprinkle the 30g (1/4 cup) of all-purpose flour over the caramelized onions and garlic. Stir well and cook for 1-2 minutes to toast the flour, which will help thicken the soup.

Pour in the 240ml (1 cup) of dry white wine. Scrape the bottom of the pot with a wooden spoon to loosen any browned bits (fond) – this adds immense flavor. Let the wine simmer and reduce by about half, which should take 3-5 minutes.

Add the 2L (8 cups) of beef broth, 4 sprigs of fresh thyme, 1 bay leaf, and 15ml (1 tbsp) of Worcestershire sauce. Bring the soup to a simmer.

Reduce the heat to low, cover the pot partially, and let the soup simmer gently for at least 30 minutes to allow the flavors to meld. Season with salt and freshly ground black pepper to taste. Remove and discard the thyme sprigs and bay leaf before serving.

While the soup simmers, prepare the baguette slices. Arrange the 2.5cm (1 inch) thick baguette rounds on a baking sheet. Toast them in a preheated oven at 190°C (375°F) for about 8-10 minutes, or until lightly golden and crisp. Alternatively, you can toast them under a broiler, watching carefully to prevent burning.

If using, add 30ml (2 tbsp) of Cognac or Brandy to the soup just before serving. This adds an extra layer of complexity. Stir well.

Ladle the hot soup into oven-safe bowls. Place one or two toasted baguette slices on top of the soup in each bowl. Generously sprinkle the 200g (7 oz) of shredded Gruyère cheese over the baguette slices, ensuring it covers them completely.

Place the soup bowls on a sturdy baking sheet and broil for 2-4 minutes, or until the cheese is melted, bubbly, and golden brown. Watch very carefully to prevent burning.

Carefully remove the soup bowls from the oven. Let them rest for a minute or two as they will be extremely hot. Serve immediately.