Desalt the pretzels and cut the thick pieces into 6 slices each. Soak the rest soak in warm water. Combine the ground meat with the egg and parsley. Wring the pretzels well and add to the meat. Season with salt and pepper. Knead well. With wet hands, form int 12 balls and press slightly flat.
Cook in oil for about 4-5 minutes on each side. Grate the horseradish and mix with the Sour Cream. Season with salt. Peel the onions, halve and cut into thin half-rings. Sauté in butter until golden brown. Spread on the pretzel slices. Top with the finished meatballs and serve garnished with horseradish cream.