Pick and clean the sorrel and scald it. Chop it finely, cook in butter till quite tender and rub it through a sieve. Boil the potatoes, rub them through a sieve, and mix with the sorrel purée. Boil the milk, add to the hot stock, stir well and add the sorrel and potato purée. Season with salt and pepper, simmer for 10 minutes and add the eggs, mixed with a little of the hot soup, and finally stir in the sour cream.