Melt 2 tablespoons butter in a medium sized skillet over medium heat. Add sliced mushrooms and chopped onion. Sprinkle mushrooms and onions with salt. Cook until the onions have become transparent and the mushrooms have released their water. Add chopped garlic, and saute for 1 to 2 minutes.
In a medium sized saucepan add chicken broth, and the mushroom mixture. Heat through. Use either a stick blender, or a food processor to process soup. Process soup until the mushrooms are chopped very fine. Add heavy cream to the soup, and heat until the soup is warm. Add truffle oil just before serving. You may want to drizzle a bit of sour cream if desired on the soup just before serving.