To begin, preheat your oven to 180°C (350°F). Slice the onions thinly, ensuring even pieces for uniform caramelization. In a large, heavy-bottomed pot, melt the butter and olive oil over medium heat. Add the sliced onions and sauté, stirring occasionally, for 25-30 minutes, allowing them to soften and turn golden brown. This slow cooking process is key to achieving the sweet, rich flavour characteristic of French onion soup.

Once the onions are deeply caramelized, add the minced garlic and cook for an additional 2 minutes, stirring continuously to prevent burning. The garlic will add a subtle depth of flavour to the soup.

Sprinkle the flour over the onions, stirring to coat them evenly. Cook for 2-3 minutes to lightly toast the flour, which will help thicken the soup and give it a smooth consistency. This technique, known as a roux, is common in French cuisine to create a velvety texture.

Deglaze the pot by pouring in the dry white wine, stirring to scrape up any browned bits from the bottom. Let the wine simmer for 3-4 minutes until it reduces slightly, intensifying its flavour. The wine adds a slight acidity, balancing the sweetness of the onions.

Add the beef stock, bay leaf, and thyme sprigs. Bring the mixture to a boil, then reduce the heat to low and let it simmer uncovered for 25-30 minutes. This allows the flavors to meld together beautifully. Adjust seasoning with salt and black pepper to taste as it simmers.

While the soup is simmering, prepare the bread. Arrange the baguette slices on a baking sheet and toast them in the preheated oven for 10-12 minutes, flipping halfway through, until golden and crispy. These croutons will add a satisfying crunch to the soup.

Once the soup has simmered and the flavors have developed, remove the bay leaf and thyme sprigs. Ladle the hot soup into oven safe bowls. Place a slice or two of toasted baguette on top of each serving, then generously sprinkle with grated Gruyère cheese.

Place the bowls under the oven broiler for 3-4 minutes, or until the cheese is melted, bubbly, and slightly browned. Carefully remove the bowls from the oven and serve immediately. The soup is best enjoyed piping hot, with the melted cheese adding a savoury richness to every bite.