01 - In a large pot, melt butter over medium heat. Add onions and celery; sauté for 3-4 minutes until softened.
02 - Stir in minced garlic and cook for 1 minute until fragrant.
03 - Add sliced mushrooms and cook for 8-10 minutes, stirring occasionally, until their liquid mostly evaporates and they develop a golden color.
04 - Pour in dry white wine if using, then simmer for 2 minutes to reduce slightly.
05 - Add vegetable stock, fresh thyme, and bay leaf. Bring to a gentle boil, reduce heat, and simmer uncovered for 15 minutes.
06 - Remove bay leaf. Use an immersion blender to purée soup until smooth, or leave some texture as preferred.
07 - Stir in heavy cream, season with salt and pepper. Heat gently for 2-3 minutes without boiling.
08 - Ladle soup into bowls, garnish with a drizzle of cream and fresh thyme sprigs, then serve hot.