Slice the scallions (both the green and the white parts), mash the garlic and cook slowly in the fat until soft, but still green. Add flour, seasoning, and part of the chicken stock and cook until thick. Purée in the blender, if desired. Return to the heat and add the mushrooms, the rest of the stock, cream, tarragon, and lemon juice, and cook for just a few minutes. Stir some of the soup into the egg yolks, and add the egg to the rest of the soup, being careful to stir quickly so that the egg will not curdle. Serve at once.