01 - Melt 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 along with a generous pinch of salt and freshly cracked black pepper to coat the mushrooms evenly.
04 - Pour in the vegetable broth and stir to combine, scraping up any browned bits from the bottom of the pot. Bring to a boil, then reduce the heat to low and let the soup simmer gently for 15 minutes to develop the flavors.
05 - For a velvety, smooth texture, use an immersion blender to purée the soup directly in the pot until it reaches your preferred consistency. For a rustic, chunky soup, skip this step entirely.
06 - Stir the heavy cream into the soup and warm gently over low heat for 2 to 3 minutes. Do not let it boil. Taste and adjust the salt and pepper as needed.
07 - Ladle the hot soup into individual bowls and garnish with freshly chopped parsley. Serve immediately with crusty bread or garlic toast alongside.