Soak the rolls in a bowl of hot water for about 15 minutes. Place a small plate on the roll to weigh it down. Then drain the water. Squeeze the bread roll very well at the edge of the bowl.
Peel onion and chop very finely. Lift out anchovies with a fork, rinse with cold water, drain and chop finely.
Put the onion, anchovies, rolls, minced meat, egg and mustard in a bowl. Season well with salt and pepper. Knead well with your hands.
With moistened hands form 8-10 meatballs from the chopping mass. Important: Place a bowl of cold water next to it and moisten your hands again and again. In this way the mince will not stick to them.
Bring 2 l salted water to the boil in a wide saucepan. Add the meatballs using a skimmer. Bring the water to the boil again. Cover the meatballs and let them simmer on low heat for 15-20 minutes.
Lift the finished meatballs out of the stock with a skimmer, drain and place on a plate. Pour the stock through a fine sieve, collecting 750 ml in a measuring cup and allow to cool for 5-10 minutes.
For the sauce, melt butter in a pot at low heat (but do not brown). Pour the flour completely into the pot and sweat it with a whisk while stirring for about 1 minute (the flour should not brown).
Gradually stir in the measured stock and cream. Bring to the boil while stirring and simmer for about 5 minutes while stirring continuously. This way the flour taste disappears and the sauce binds.
Add capers, caper liquid and lemon juice to the béchamel sauce. Season with salt, pepper and sugar. Heat Königsberger meatballs for 5-10 minutes in the sauce. Serve with parsley potatoes (mix finely chopped parsley with boiled boiled potatoes) and beetroot salad.