01 - Melt the butter with olive oil in a large heavy-bottomed pot over medium heat. Add the diced onion and sauté, stirring occasionally, 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 every couple of minutes, until the mushrooms are deeply golden brown and most of their released moisture has cooked off.
03 - Stir in the dried thyme, a generous pinch of kosher salt, and freshly cracked black pepper. Toss to coat the mushrooms evenly and let the herbs bloom for about 30 seconds.
04 - Pour in the vegetable broth and stir well, scraping up any browned bits from the bottom of the pot. Bring to a gentle boil, then reduce the heat to medium-low and let the soup simmer uncovered for 15 minutes to develop the flavors.
05 - For a silky, smooth texture, use an immersion blender to purée the soup directly in the pot until it reaches your preferred consistency. For a rustic, chunky texture, skip this step or purée only half of the soup.
06 - Stir in the heavy cream and heat gently over low for 2 to 3 minutes, just until warmed through. Do not allow the soup to boil after adding the cream. Taste and adjust the salt and pepper as needed.
07 - Ladle the hot soup into warmed bowls. Sprinkle each portion with chopped fresh parsley and serve immediately alongside crusty bread or garlic toast.