01 - Heat olive oil and butter in a large pot over medium heat. Add chopped onion, carrot, and celery. Sauté for 5 minutes until vegetables are softened and fragrant.
02 - Stir in minced garlic and cook for 1 minute until aromatic, being careful not to brown the garlic.
03 - Increase heat to medium-high. Add sliced mushrooms and thyme. Cook for 8-10 minutes, stirring occasionally, until mushrooms are golden brown and most of their liquid has evaporated.
04 - Pour in vegetable broth and add bay leaf. Bring to a boil, then reduce heat to low. Simmer uncovered for 15 minutes to allow flavors to meld.
05 - Remove and discard bay leaf. Using an immersion blender, purée the soup until completely smooth. For a rustic texture, blend partially, leaving some mushroom pieces intact.
06 - Stir in heavy cream and heat gently for 2-3 minutes. Do not let the soup boil after adding cream. Season generously with salt and freshly ground black pepper to taste.
07 - Ladle hot soup into bowls. Garnish with fresh thyme sprigs if desired. Serve immediately with crusty bread.