Heat the olive oil and butter in a large pot over medium heat.
Add the chopped onions and sauté until translucent.
Stir in the minced garlic and cook for another minute.
Add the sliced mushrooms and cook until soft, about 5-7 minutes.
Sprinkle in thyme, salt, and pepper, stirring well.
Pour in the vegetable or chicken broth and bring to a simmer.
Let it simmer for 15 minutes to develop flavors.
Use an immersion blender to puree the soup until smooth.
Return the soup to the pot and stir in the heavy cream.
Heat through, adjust seasoning, and serve hot, garnished with fresh parsley.