Mushroom Soup

1) In a kettle cook the onion in the butter over moderately low heat, stirring occasionally, until it is translucent. 2) Add the carrots, the parsley roots, and 2 cups of the chicken stock mixture. 3) Cover partially, for 50 minutes to 1 hour, or until the liquid is almost evaporated. 4) Stir in the mushrooms and cook the mixture over moderate heat, stirring occasionally, until the liquid the mushrooms give off is evaporated. 5) Add the remaining 2 cups of stock and bring the mixture to a simmer. 6) In a bowl whisk together the sour cream and the flour and whisk in the yolks, the paprika, and salt to taste. 7) Add 1 cup of the hot liquid in a stream, whisking, and stir the mixture into the kettle. 8) Cook the soup at a bare simmer, stirring occasionally, for 5 minutes and sprinkle it with parsley.