Wash the sorrel well and chop it up. Heat the butter in a saucepan and just soften the sorrel and onion in it. Shake the flour over the vegetables and mix well. Let it cook for about 1 minute. Meanwhile bring the stock to the boil, then add to the pan. Add the breadcrumbs, season to taste, and bring to the boil, then simmer for about 1 hour covered. (It can be liquidized at this point, but needn't be.) Beat the egg yolks with the cream and add a little of the hot soup to the mixture, stirring well; then add gradually to the soup pot, stirring well, over the heat, but being careful not to let it boil.