Caramelize the Onions:
In a large pot or Dutch oven, melt the butter over medium heat. Add the sliced onions and cook, stirring occasionally, for 15-20 minutes until the onions are golden brown and caramelized. This step is key to developing the flavor.

Add the Mushrooms and Garlic:
Add the sliced mushrooms to the pot and cook for an additional 5-7 minutes until they release their moisture and begin to brown. Stir in the minced garlic and cook for another minute until fragrant.

Simmer the Soup:
Sprinkle the flour over the onions and mushrooms (if using) and stir to combine. Gradually add the beef broth while stirring to prevent lumps. Add the Worcestershire sauce, thyme, and bay leaf. Bring the soup to a boil, then reduce the heat to low and simmer for 20-25 minutes, allowing the flavors to meld together. Season with salt and pepper to taste.

Add the Cream:
Stir in the heavy cream and let the soup simmer for another 5 minutes. If you prefer a thicker soup, you can blend a portion of it using an immersion blender or regular blender. If the soup is too thick, add a bit more broth or water to reach your desired consistency.

Prepare the Bread and Cheese Topping:
While the soup simmers, preheat your oven’s broiler. Place the slices of baguette on a baking sheet and toast under the broiler for 1-2 minutes, until golden. Remove from the oven and top each slice of bread with a generous amount of grated Gruyère cheese.

Serve:
Ladle the soup into bowls, and place a slice of cheese-topped bread on top of each bowl. Place the bowls under the broiler for 1-2 minutes, or until the cheese is melted and bubbly. Garnish with chopped fresh parsley.