01 - Heat olive oil and butter in a large pot over medium heat. Add onions and celery, sauté until softened, about 5 minutes.
02 - Add garlic and cook for 1 minute until fragrant.
03 - Add mushrooms and cook, stirring occasionally, until they release their juices and begin to brown, about 8-10 minutes.
04 - Add white wine if using and simmer for 2-3 minutes until reduced.
05 - Stir in thyme and bay leaf. Pour in vegetable broth and bring to a gentle simmer. Cook uncovered for 15 minutes.
06 - Remove bay leaf. Using an immersion blender, purée soup to desired consistency, leaving some texture or making it silky smooth.
07 - Stir in heavy cream. Season with salt and pepper to taste. Simmer gently for 2-3 more minutes.
08 - Ladle into bowls and garnish with fresh parsley and a sprig of thyme.