Melt the butter in a medium-sized skillet over medium heat. Add the onions and mushrooms, season with salt, and sauté until the onions have become translucent and the mushrooms have released their water. Add the garlic and sauté for 1 to 2 minutes.

Transfer the onions and mushrooms to a medium-sized saucepan over medium heat. Add the chicken broth and cook until heated through.

Use an immersion blender or food processor to process the soup until the mushrooms are very finely chopped.

Add the heavy cream and heat until the soup is warm. Add the truffle oil just before serving.

To serve, ladle the soup into bowls and drizzle with sour cream.