Caramelize the onions: In a large pot, melt the butter over medium heat. Add sliced onions, garlic, bay leaves, thyme, salt, and pepper. Cook slowly, stirring often, until the onions are soft and deeply caramelized; about 25 minutes.

Deglaze with wine: Pour in the red wine and bring to a boil. Reduce heat and simmer until most of the liquid evaporates and the onions look jammy, about 5 minutes. Remove the bay leaves and thyme sprigs.

Add flour: Sprinkle flour over the onions and stir to coat evenly. Reduce the heat to medium low and cook for about 10 minutes to remove the raw flour taste.

Add broth: Pour in the beef broth, stir, and bring the soup back to a simmer. Cook for another 10 minutes. Taste and adjust seasoning with salt and pepper as needed.

Prepare the cheesy toasts: Preheat your broiler. Arrange baguette slices on a baking sheet, sprinkle with grated Gruyère, and broil until bubbly and golden brown; about 3–5 minutes.

Serve: Ladle the hot soup into bowls, then top each serving with cheesy toasts.

Alternative method: You can also float 2 slices of bread topped with cheese directly on the soup and broil until melted and golden.