01 - In a large pot, melt the butter (and olive oil, if using) over medium heat. Add the onion and celery; cook until softened, about 5 minutes.
02 - Stir in the garlic and cook for 1 minute until fragrant.
03 - Add the sliced mushrooms and thyme. Cook, stirring occasionally, until the mushrooms release their juices and become golden, about 10 minutes.
04 - Season with salt and pepper.
05 - Pour in the vegetable broth, bring to a boil, then reduce heat and simmer uncovered for 15 minutes.
06 - Remove from heat. Using an immersion blender, puree the soup until smooth (or leave some texture if desired). Alternatively, blend in batches in a countertop blender.
07 - Stir in the heavy cream and heat gently for 2–3 minutes. Do not boil.
08 - Taste and adjust seasoning if necessary.
09 - Ladle into bowls and garnish with fresh parsley.