Pour in the 1 liter beef stock and bring it to a boil, then reduce to a simmer. Let the vegetables cook gently while you prepare the meatballs. In a separate bowl, combine the 1/4 kg beef, 2 tablespoons rice, 2 tablespoons breadcrumbs, 1 egg, and chopped parsley. Mix everything well, then roll the mixture into small meatballs, roughly 1-inch in diameter. If you're short on time, you can prep these ahead and even freeze them.
Carefully drop the meatballs into the simmering soup and cook for about 20 minutes, or until the rice inside the meatballs is fully cooked. Season the soup with lemon juice, salt, and pepper to taste. If you like, you can slowly whisk in a beaten egg at this stage, stirring continuously to create fine strands in the broth.
Stir in the remaining fresh parsley leaves just before serving. This soup is traditionally served with a dollop of sour cream on top, if you wish.