Prepare the onions: Peel the yellow onions and slice them thinly, about 3mm (1/8 inch) thick. It's helpful to use a mandoline for uniform slices, but a sharp knife works well. Aim for consistency for even caramelization.

Start caramelizing the onions: In a large, heavy-bottomed pot or Dutch oven, melt the unsalted butter (100g or 7 tbsp) with the olive oil (30ml or 2 tbsp) over medium-low heat. Add the thinly sliced onions and stir to coat them evenly with the fat. Add the granulated sugar (15g or 1 tbsp) to help with caramelization.

Caramelize the onions slowly: Cook the onions, stirring occasionally, for at least 45 minutes to 1.5 hours, or until they are deeply golden brown and significantly reduced in volume. The key is low and slow heat. If they start to brown too quickly, reduce the heat to low. Scrape the bottom of the pot frequently to prevent sticking and burning. This slow process develops the sweet, complex flavor characteristic of French onion soup.

Deglaze with wine: Once the onions are deeply caramelized, sprinkle the all-purpose flour (40g or 1/3 cup) over them and stir well. Cook for 1-2 minutes to cook out the raw flour taste. Pour in the dry white wine (240ml or 1 cup) and scrape the bottom of the pot to loosen any browned bits (fond). Let the wine simmer and reduce by about half, about 5 minutes.

Add broth and aromatics: Pour in the beef broth (2L or 8.5 cups). Add the fresh thyme sprigs (4) and the bay leaf (1). Bring the soup to a simmer, then reduce the heat to low, cover partially, and let it gently simmer for at least 30 minutes to allow the flavors to meld.

Season the soup: Remove and discard the thyme sprigs and bay leaf. Stir in the Worcestershire sauce (15ml or 1 tbsp). Taste the soup and season generously with salt and freshly ground black pepper. Remember that the cheese will add saltiness, so be mindful.

Prepare the croutons: While the soup simmers, preheat your broiler or oven to 200°C (400°F). Arrange the baguette slices (6) on a baking sheet. Toast them lightly on both sides until golden brown and crisp. This usually takes about 5-10 minutes, flipping halfway through. Once toasted, rub each slice with the cut side of a halved garlic clove.

Assemble and broil: Ladle the hot soup into oven-safe bowls. Place one toasted baguette slice on top of the soup in each bowl. Generously sprinkle the grated Gruyère cheese (200g or 7 oz) over the baguette, ensuring it covers the bread and spills slightly onto the soup's edge. This helps seal the bread.

Melt the cheese: Carefully place the soup bowls on a sturdy baking sheet (this makes them easier to handle and catches any drips). Broil for 2-4 minutes, or until the cheese is melted, bubbly, and lightly golden brown. Watch closely to prevent burning.

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