Soak toast in cold water. Peel onions and dice 1 1/2 finely. Squeeze the toast well. Add the bread, egg and 2/3 of the onion cubes to the mince. Season with salt, pepper and mustard and knead well.
Form dumplings of different sizes with moistened hands. Bring lightly salted water to the boil and add the larger dumplings first. Let them simmer for about 10 minutes. After 5 minutes, add the remaining meatballs and cook them.
Meanwhile, put rice in 500 ml of boiling salted water, cover and allow to swell over a low heat for 15-20 minutes. Wash the tomatoes, dab dry and cut into slices. Cut the remaining onion half into fine slices.
Mix vinegar with salt and pepper and fold in the oil. Mix tomatoes, onions and vinaigrette. Place the prepared meatballs in a bowl and keep warm. Measure 1/8 litre of the meat stock and stir in clear stock.
Set aside. Heat the fat in a pot and fry the remaining onion cubes in it until transparent. Deglaze with cream and meat stock and bring to the boil. Mix cornflour and 2-3 tablespoons of cold water and thicken the sauce.
Simmer for about 1 minute and remove from the heat. Mix yoghurt and egg yolk, then stir into the hot sauce. Add capers and lemon zest to the sauce, season with salt, pepper and lemon juice.
Arrange rice, Königsberger meatballs, sauce and tomato salad on plates. Sprinkle with chives as desired and garnish with lemon and chives.