Note: You may be tempted to skip the egg yolk step at the end, but don't. The method--known as "tempering eggs"--allows you to add them to a hot liquid without ending up with scrambled eggs. The addition of egg yolks is used here as an emulsifier, binding the liquids with the fats in the soup, stabilizing the ingredients and the flavor, and giving the soup a wonderfully silky texture. It's super easy, quick, and works like magic.