Melt butter in a large pot over medium heat. Add onion and cook until softened, about 5 minutes.

Add garlic and cook for 1 minute more.

Add mushrooms and cook until browned and softened, about 8-10 minutes.

Pour in vegetable broth, add thyme and bay leaf. Bring to a boil, then reduce heat and simmer for 10 minutes.

Remove bay leaf. Carefully transfer soup to a blender (or use an immersion blender) and blend until smooth.

Return soup to the pot and stir in heavy cream. Heat through, but do not boil.

Season with salt and pepper to taste. Serve hot.