Caramelize the Onions
1-Heat the butter and olive oil in: Heat the butter and olive oil in a large, heavy-bottomed pot over medium heat.
2-Add the sliced onions, thyme leaves (removed: Add the sliced onions, thyme leaves (removed from their sprigs), and bay leaf. Stir to coat the onions in the butter and oil.
3-Reduce the heat to low and cook: Reduce the heat to low and cook for about 45 minutes, stirring occasionally at first and more frequently as the onions begin to brown. Season with salt and continue cooking until the onions are deeply golden brown, soft, and caramelized. If they begin to stick, add a splash of water and scrape up the browned bits.

Make the Soup
4-Pour in the white wine, stirring to: Pour in the white wine, stirring to scrape up any browned bits from the bottom of the pot. Simmer for 2-3 minutes to reduce slightly.
5-Add the garlic and cook for a: Add the garlic and cook for a further 3-5 minutes until fragrant. Sprinkle in the flour and stir to coat the onions.
6-Add the beef broth and bring to: Add the beef broth and bring to a gentle boil. Reduce the heat and simmer for 20-30 minutes to allow the flavors to develop. Taste and adjust the seasoning with salt and black pepper.

Prepare the Topping
7-Preheat the broiler: Preheat the broiler. Ladle the soup into four oven-safe bowls and top each with a slice of toasted baguette.
8-Divide the grated Gruyère evenly between the: Divide the grated Gruyère evenly between the bowls, covering the bread generously.

Broil and Serve
9-Place the bowls on a sturdy baking: Place the bowls on a sturdy baking sheet and broil for 3-5 minutes, until the cheese is melted, bubbling, and golden brown. Watch closely to prevent burning.
10-Garnish with additional fresh thyme, if desired,: Garnish with additional fresh thyme, if desired, and serve immediately. Be careful, as the bowls will be very hot.