Heat the olive oil and butter in a large pot over medium heat.
Add the diced onion and cook until it becomes translucent, about 5 minutes.
Add the sliced mushrooms and minced garlic to the pot. Cook until the mushrooms are browned and have released their moisture, about 10 minutes.
Pour in the vegetable broth and bring the mixture to a boil. Reduce heat and let it simmer for 15 minutes.
Using an immersion blender, blend the soup until smooth. Alternatively, you can transfer the soup to a blender in batches.
Stir in the heavy cream and let it simmer for another 5 minutes. Season with salt and pepper to taste.
Serve hot, garnished with freshly chopped parsley and sliced mushrooms.