Heat olive oil in a large pot over medium heat. Add the onions and garlic, cooking for about 3-4 minutes until softened and fragrant.
Add the sliced portabella mushrooms and cook until tender, about 7-8 minutes.
Stir in the thyme, then pour in the broth. Bring to a simmer and cook for 10 minutes to allow the flavors to meld.
Using an immersion blender or regular blender, blend the soup until smooth and creamy.
Return to heat, stir in the heavy cream, and season with salt and pepper. Simmer for another 5 minutes to combine.
Serve hot, garnished with fresh parsley.