Sauté the Onions:
Begin by heating the unsalted butter and olive oil in a large pot over medium heat. Introduce the thinly sliced onions, sautéing them for approximately 15-20 minutes. Stir frequently until they achieve a caramelized, golden brown color.
Add Mushrooms and Garlic:
Incorporate the sliced mushrooms and minced garlic into the pot, continuing to cook for an additional 5-7 minutes. This step allows the mushrooms to become tender and release their moisture.
Add Broth and Seasoning:
Pour in the vegetable or beef broth, and add the dried thyme and Worcestershire sauce. Bring the mixture to a simmer and let it cook for around 10 minutes, allowing the flavors to meld together. Season with salt and pepper according to your preference.
Stir in Cream:
Lower the heat and mix in the heavy cream, ensuring it is well combined. Gently heat the soup until it is warmed through.
Prepare to Serve:
If using baguette slices, toast them in the oven until they reach a golden brown color. Top each slice with grated cheese.
Serve the Soup:
Ladle the soup into bowls, garnishing each with a toasted baguette slice and melted cheese, if desired. Finish with a sprinkle of chopped parsley.