Saute the Aromatics: Heat the butter and olive oil in a large Dutch oven or soup pot over medium heat. Once the butter has melted, add the finely diced onion. Cook for 4 to 5 minutes until the onion is soft and translucent.

Cook the Mushrooms: Add the sliced mushrooms to the pot. Stir well to coat them in the butter mixture. Let them cook undisturbed for a few minutes to develop a nice brown color, then stir occasionally until they have released their moisture and cooked down significantly (about 8 to 10 minutes).

Add the Garlic: Stir in the minced garlic and fresh thyme leaves. Cook for 1 additional minute until highly fragrant.

Create the Roux: Sprinkle the all-purpose flour evenly over the mushroom mixture. Stir continuously for 1 to 2 minutes. This cooks out the raw flour taste and creates a thick paste that will thicken the soup.

Simmer the Soup: Slowly pour in the vegetable broth, stirring constantly to ensure no lumps form from the flour. Add the salt and black pepper. Bring the soup to a gentle boil, then reduce the heat to low. Cover the pot and let it simmer for 10 to 15 minutes to allow the flavors to meld together.

Blend (Optional): If you prefer a smoother soup, use an immersion blender to puree the soup right in the pot until it reaches your desired consistency. You can blend it completely smooth or leave half of it chunky for texture. If using a traditional blender, carefully blend in small batches and return to the pot.

Finish with Cream: Remove the pot from the heat. Stir in the heavy whipping cream until fully combined. Taste the soup and adjust the salt and pepper if necessary.

Serve: Ladle the hot soup into bowls. Garnish with a sprinkle of fresh chopped parsley and a few extra sautéed mushroom slices if desired. Serve immediately with crusty bread for dipping.