Mix egg, water, flour and salt together. Rest 10 minutes in a warm bowl. Mix sauerkraut with chopped onion. Mix potatoes with cottage cheese. Roll out dough and cut into 12 rounds. Fill half with each mixture. Fold in half and seal edges with water. Drop in salted boiling water four minutes. Remove with a slotted spoon and brown in oil. Serve well salted. These freeze very well after boiling.