Marinate the apple cubes with the lemon juice. Sear the onions and apple cubes with the sugar in butter, add the potatoes and deglaze with the white wine. Let simmer for 5 minutes. Then pour in the broth and simmer for 20 minutes. Finally stir in the horseradish and season with salt and pepper.
Place the beef and apple slices on the soup for decoration, sprinkle with the chopped parsley - serve hot.