For the Königsberger Klopse, mix minced meat, onion, egg, mustard, parsley, salt and pepper. Add breadcrumbs and knead into a smooth mass.
Form small dumplings and cook in the vegetable soup until they float. Let them stand for a while and take them out. Lightly brown the butter, stir in the flour, remove from heat and stir through with about 3/4 of the soup to make a creamy sauce.
Stir in the sour cream and the yolk whisked with a little milk and mix in the capers. Season the Königsberger Klopse if necessary.
Let the dumplings steep in the sauce for a while.