Heat olive oil in a large pot over medium heat. Add sliced onions and cook until caramelized, about 15 minutes.

Add sliced mushrooms, garlic, carrots, and celery. Cook for an additional 10 minutes until softened.

Pour in the vegetable broth and add thyme, salt, and pepper. Bring to a boil, then reduce heat and simmer for 20 minutes.

Using a blender, puree the soup until smooth. Return to the pot and stir in the heavy cream.

Ladle the soup into bowls, garnish with grated Gruyère cheese and chopped parsley. Serve hot.