In a large heavy-bottomed pot or Dutch oven, melt the butter with the olive oil over medium heat. Add the thinly sliced onions and toss them thoroughly to coat in the fat.

Cook the onions slowly, stirring every 5-10 minutes, until they soften and turn a deep, rich mahogany brown. This essential caramelization process should take about 40 to 45 minutes; reduce the heat to medium-low if they start to burn or stick.

Once the onions are deeply caramelized, stir in the minced garlic and cook for 1 minute until highly fragrant. Pour in the dry white wine to deglaze the pot, using a wooden spoon to scrape up all the delicious browned bits from the bottom.

Pour in the beef broth, then add the fresh thyme leaves, bay leaf, a pinch of salt, and black pepper. Bring the soup to a gentle simmer and let it cook uncovered for 15 minutes to allow the rich flavors to meld beautifully.

While the soup is simmering, preheat your oven's broiler. Arrange the baguette slices on a baking sheet and toast them under the broiler for 1-2 minutes per side until golden brown and crispy.

Remove and discard the bay leaf from the soup. Ladle the hot broth and onions into four oven-safe crocks or bowls placed on a sturdy baking sheet, then top each bowl with 1-2 toasted baguette slices and a generous handful of shredded Gruyère cheese.

Broil the bowls for 2-3 minutes, watching very closely, until the cheese is bubbling, completely melted, and slightly browned on top. Carefully remove from the oven and let cool for just a minute before serving with a crisp side salad for a complete meal.