01 - In a large pot over medium heat, melt the butter with the olive oil.
02 - Add the onions and celery; sauté for 4–5 minutes until softened.
03 - Stir in the garlic and cook for 1 minute until fragrant.
04 - Add the sliced mushrooms and cook, stirring occasionally, for 8–10 minutes until mushrooms are golden and most of their moisture has evaporated.
05 - Deglaze the pot with white wine, scraping up any browned bits, and let it reduce for 2 minutes.
06 - Add the thyme, bay leaf, and vegetable broth. Bring to a boil, then reduce heat and simmer uncovered for 15 minutes.
07 - Remove the bay leaf from the soup.
08 - Using an immersion blender (or in batches with a blender), purée the soup until smooth or your desired texture.
09 - Return the soup to low heat, stir in the cream, and heat gently for 2–3 minutes. Season with salt and pepper to taste.
10 - Serve hot, garnished with parsley and a sprinkle of fresh thyme, if desired.