Soak rolls in cold water. Peel and finely chop the onion. Knead minced meat, squeezed out roll, onion and egg. Season with salt, pepper and mustard. Form 12 dumplings with moistened hands.
Bring slightly salted water to the boil. Add the dumplings and cook over a low heat for 10 minutes. Remove the dumplings. Pour stock through a sieve and measure 1/2 litre. Melt the fat in a pot.
Sweat flour in it. Add the stock while stirring constantly and simmer for 5 minutes at low heat. Drain the capers. Mix cream and egg yolk and stir into the sauce, do not let it boil anymore! Season to taste with salt, pepper and lemon juice.
Add dumplings and capers to the sauce. Arrange dumplings on plates. Boiled potatoes and beetroot taste good with them.