Heat olive oil in a large pot over medium heat. Add the onion and cook, stirring occasionally, until golden and caramelized, about 20 minutes. Remove and set aside.

In the same pot, add butter (if using), and sauté the garlic and mushrooms until softened and browned, about 10 minutes.

Return the onions to the pot, add the thyme, and pour in the vegetable broth. Bring to a boil, then reduce to a simmer for 15 minutes.

Blend the soup using an immersion blender or regular blender until smooth.

Stir in the heavy cream, season with salt and pepper, and heat for another 5 minutes.

Serve hot, garnished with extra cream or fresh herbs if desired.