01 - Melt the butter in a large pot over medium heat. Add the onions, carrot, celery, and garlic; sauté until softened, about 5 minutes.
02 - Add the mushrooms and cook, stirring occasionally, until they release their moisture and begin to brown, about 8–10 minutes.
03 - Stir in the thyme and bay leaf. Pour in the vegetable broth and bring to a simmer. Cover and cook for 15 minutes.
04 - Remove the bay leaf. Use an immersion blender to purée the soup until smooth (or leave some texture if desired).
05 - Stir in the milk and cream. Heat gently until warmed through, but do not boil. Season with salt and pepper to taste.
06 - Serve hot, garnished with extra thyme and a drizzle of cream if desired.