Prepare the Onions: Peel the onions, halve them from root to tip, and then slice them thinly (about 3-5mm thick). This uniform size helps them cook evenly.

Begin Sautéing: In a large, heavy-bottomed Dutch oven or stockpot, melt the butter with the olive oil over a medium-low heat. Add the sliced onions and stir to coat them completely. Cook, stirring occasionally, for 15-20 minutes until they have softened and become translucent.

Caramelise the Onions: Reduce the heat to low, add the optional sugar, and continue to cook for a further 40-50 minutes. You need to stir every 5-10 minutes to prevent them from sticking and burning. I find that scraping the bottom of the pot with a wooden spoon is essential here. The onions will slowly transform from pale, to golden, to a deep, rich mahogany brown. They should be very soft and jam-like. This is the most important step! For an in-depth look at this process, Serious Eats has a great article on the science behind it.

Build the Flavour Base: Once the onions are deeply caramelised, stir in the minced garlic and cook for 1 minute until fragrant. Sprinkle over the plain flour and stir continuously for another minute. This will help to slightly thicken the soup.

Deglaze and Simmer: Pour in the white wine to deglaze the pot. Scrape up any browned bits from the bottom – this is where so much flavour is. Let the wine bubble and reduce by about half, which should take 2-3 minutes.

Create the Broth: Add the beef stock, thyme sprigs, and bay leaf. Bring the soup to a gentle simmer. Reduce the heat to low, partially cover the pot, and let it simmer gently for at least 30 minutes to allow the flavours to meld together.

Finish the Soup: Remove the thyme sprigs and bay leaf. Stir in the brandy or cognac if you're using it. Season generously with salt and freshly ground black pepper to taste.

Prepare the Croutons: While the soup simmers, preheat your grill to high. Arrange the baguette slices on a baking sheet and toast them under the grill for 1-2 minutes on each side until golden and crisp.

Assemble and Grill: Ladle the hot soup into oven-safe bowls. Place one or two toasted baguette slices on top of the soup in each bowl. Generously cover the bread with the grated Gruyère cheese. Carefully place the bowls on a sturdy baking sheet and put them under the hot grill for 3-5 minutes, or until the cheese is completely melted, bubbling, and has developed golden-brown spots. Serve immediately, but with a warning that the bowls are extremely hot!