For the meatballs, soak rolls in cold water for about 10 minutes. Peel and finely dice the onion. Put minced meat, onion, egg and mustard in a bowl. Squeeze out the buns and add. Season with 1 level tsp salt and 1⁄2 tsp pepper. Knead everything with your hands. Form approx. 12 meatballs from the mixture with moistened hands. 3⁄4 Bring l salted water to the boil in a pot.
Peel and wash the potatoes and cook them in boiling salted water for 20-25 minutes. Put the meatballs into the lightly boiling salted water and let them simmer in it at medium heat for about 12 minutes.
For the sauce, chop the capers very finely. Remove the meatballs from the cooking stock with a skimmer. Pour the stock through a sieve and measure 1⁄2 l. Melt butter in a pot. Dust with flour and sauté while stirring until golden brown. While stirring, deglaze with the stock, bring to the boil and simmer for about 5 minutes.
Wash parsley and shake dry, pluck the leaves and cut into fine strips. Whisk the egg yolk and cream. Stir the capers into the sauce. Remove from heat and stir the egg yolk cream into the sauce, which is no longer boiling. Season to taste with salt, pepper and lemon juice. Add the meatballs and let them simmer for a short time. Drain the potatoes. Serve with meatballs and sauce. Sprinkle with parsley.