01 - Melt the butter with olive oil in a large heavy-bottomed pot over medium heat. Add the chopped onion and sauté until translucent and softened, about 3 to 4 minutes.
02 - Add the minced garlic and sliced mushrooms to the pot. Cook for 8 to 10 minutes, stirring occasionally, until the mushrooms are deeply browned and most of their released moisture has evaporated.
03 - Stir in the dried thyme, a generous pinch of salt, and freshly ground black pepper until evenly distributed throughout the mushroom mixture.
04 - Pour in the vegetable broth and bring to a gentle boil. Reduce the heat to low and let the soup simmer uncovered for 15 minutes, allowing the flavors to meld together.
05 - For a silky, smooth texture, use an immersion blender to purée the soup directly in the pot until you reach your preferred consistency. For a rustic version, leave the soup chunky or blend only half of it.
06 - Stir in the heavy cream and warm gently over low heat for 2 to 3 minutes. Do not let it come to a boil. Taste and adjust the salt and pepper as needed.
07 - Ladle the hot soup into warmed bowls and garnish with freshly chopped parsley. Serve immediately alongside crusty bread or garlic toast.