In a large pot, melt the butter over medium heat. Add the chopped onion and garlic and sauté for 2-3 minutes until softened.

Add the sliced mushrooms to the pot and cook for 5-7 minutes, stirring occasionally, until they release their moisture and soften.

Pour in the vegetable broth and bring the mixture to a boil. Reduce the heat and simmer for 10 minutes, allowing the flavors to blend.

Use an immersion blender to blend the soup until smooth, or transfer it to a blender in batches.

Stir in the heavy cream and thyme, and cook for another 5 minutes, until the soup is heated through.

Season with salt and black pepper to taste.

Garnish with fresh parsley before serving, if desired.