Pick and wash the sorrel thoroughly, and cook, like spinach, in boiling salted water. When quite tender remove from the saucepan, drain thoroughly, and rub through a sieve. Put this purée into a saucepan with 1 oz. of butter, and stir well, simmering for about 10 minutes and seasoning with salt and pepper. Then add the rest of the butter, divided into small pieces, and the cream gradually, stirring and mixing well. Like the purée of potatoes, this should be very creamy and perfectly smooth.