Pour the vinegar into a pot with a good 3/4 l water. Clean the vegetables and cut them into large pieces. Add them to the vinegar water together with the spices, salt and sugar. Boil the stock for 5 minutes and let it cool down.
Wash the meat and place it in the stock, it must be completely covered with liquid. Covered, leave to stand in the refrigerator for 3 days and turn once a day.
Preheat the oven to 200°C. Remove the meat from the marinade and pat dry. Season all around with pepper. Heat the lard in a roaster and fry the meat on both sides.
Pour the marinade through a sieve into a bowl. Fry the vegetables and add the tomato paste. Deglaze with half of the stock and cover the roast in the oven (fan oven 180°C) for 2 hours (turn over and add some stock).
Add bread edges after 1 hour.
Remove the roast. Peel off any remaining meat on the edge of the roaster, pour the sauce through a sieve and stir in the sour cream. Cut the sauerbraten open. Add sauce extra and serve with the spaetzle.