Pick over, rinse and finely chop the sorrel, and reserve.
Cook the diced potato in the stock till really soft, about 20 minutes. Transfer to a liquidiser and whizz till smooth. Stir in the sorrel, return the mixture to the pan and bring back to the boil, taste, and season with salt and pepper. Remove from the heat and stir in the cream, then reheat till just below boiling. Finish with a