Peel the onions, quarter them lengthways and cut into strips. Put the sugar, vinegar, red wine, bay leaves, juniper berries and mustard seeds in a saucepan and cook for about 1 minute until the sugar has dissolved. Then let it cool down.
Dab some of the oil from the matjes fillets with kitchen paper, cut them into bite-sized pieces and put them together with the cooled stock in a sealable bowl. The fish pieces should all be well covered with the stock at the end. Then put the sealed bowl in the refrigerator overnight. But check again until you have eaten and press protruding parts into the brew.
For the meal, boil potatoes with the skin - adapted to hunger - in salted water until cooked. Meanwhile, mix the quark and sour cream into the dip and season with pepper, salt and a pinch of sugar.
Then peel the skin off the potatoes if you like (we like to eat them with you) and serve with matjes and dip.
Since the rest period takes place overnight, none is given here.