Sauté the Aromatics
In a large pot, heat the butter and olive oil over medium heat. Add the chopped onion and sauté for 3-4 minutes until soft and translucent. Add the garlic and cook for another 1-2 minutes until fragrant.
Cook the Mushrooms
Add the sliced mushrooms to the pot and cook, stirring occasionally, for 7-10 minutes until the mushrooms are browned and softened. If you like a thicker soup, you can sprinkle in 1 tablespoon of flour and cook for an additional 1-2 minutes to create a roux.
Add Broth and Seasoning
Pour in the vegetable or chicken broth, and add the thyme and bay leaf. Bring the soup to a simmer and cook for 15-20 minutes, allowing the flavors to blend together.
Blend the Soup
Remove the bay leaf. If you prefer a smooth soup, use an immersion blender to blend the soup directly in the pot until smooth, or transfer the soup in batches to a regular blender. Leave a few chunks of mushrooms for texture if desired.
Add Cream and Adjust Seasoning
Return the soup to the heat and stir in the heavy cream. Simmer for another 5-7 minutes, stirring occasionally. Season with salt and pepper to taste.
Serve
Ladle the soup into bowls and garnish with fresh parsley. Serve with warm, crusty bread or a side salad for a complete meal.