Heat a large pot or Dutch oven over medium heat. Add the butter. Once melted, add the onions, bay leaves, and thyme, and season with salt and pepper. Cook, stirring frequently, until the onions are soft and caramelized, about 30 minutes. During the last 5 minutes, add the garlic. (We don't want to add it in the beginning because it can burn easily and get bitter).

Add the wine, raise heat to high and bring to a boil. Once boiling, reduce the heat and simmer until the wine has evaporated, about 2-3 minutes. Discard the bay leaves and thyme sprigs. Add the flour and stir until onions are coated. Reduce heat slightly and cook for another 2-3 minutes, stirring frequently to ensure the flour doesn't burn.

Add the beef broth, raise heat to high, and bring the soup to a boil. Once boiling, reduce heat slightly and simmer for 15 minutes. Season to taste with salt and pepper.

Preheat your broiler to high. While the soup is simmering, make the mushrooms. Heat a large skillet over medium-high heat and add the butter and olive oil. Once butter has melted, add the garlic and sauté for 30 seconds or until fragrant. Add the mushrooms and sauté for 10 minutes, stirring occasionally, or until mushrooms are golden brown. Add the thyme and season with salt and pepper.

Arrange the baguette slices on a baking sheet in a single layer. Add a spoonful or two of mushrooms to each slice, followed by a mound of the Gruyere cheese and a sprinkle of parmesan. Broil until bubbly and golden brown, about 2-3 minutes. Watch carefully because they can burn quickly!

Ladle the soup in bowls and float the cheesy mushroom toasts on top! Serve with extra parmesan cheese and fresh thyme.