01 - Melt butter in a large pot over medium heat. Add onions, carrot, celery, and garlic. Sauté until softened, approximately 5 minutes.
02 - Add sliced mushrooms to the pot. Cook, stirring occasionally, until mushrooms release moisture and begin to brown, about 8-10 minutes.
03 - Stir in fresh thyme and bay leaf. Pour in vegetable broth and bring to a simmer. Cover and cook for 15 minutes to develop flavors.
04 - Remove bay leaf. Use immersion blender to purée soup until completely smooth, leaving slight texture if preferred.
05 - Stir in whole milk and heavy cream. Heat gently until warmed through, taking care not to boil. Season with salt and pepper to taste.
06 - Ladle hot soup into bowls. Garnish with additional thyme sprigs and cream drizzle if desired.