First cut the onion into small cubes and grate the beetroot finely. Then cut up the beef, a little smaller than goulash.
Sear the meat in clarified butter in a saucepan, add the onions and sweat until translucent. Season to taste with salt, pepper and paprika. Add 1 - 2 large heaped tablespoons of mustard (depending on taste). Then add the beetroot with the juice. Now pour in the beef stock / stock.
Bring everything to a boil over medium heat. Finally, add sugar to taste. Spread the soup in deep plates and put a hazelnut-sized dollop of sour cream in the center.