For the Erdäpfelschneck cook the potatoes, peel and press through the potato press. Mix together with flour, semolina, the room-warm butter, egg yolk and a little salt to a potato dough. Fry the chopped onions in a little hot oil or butter.
Roll out the dough into a rectangle on a floured work surface and sprinkle with the roasted onions, grams and chopped parsley. Gently roll up and wrap in a cloth. Close the ends tightly.
Place in a saucepan of boiling salted water and cook for 20-30 minutes. Lift out the Erdäpfelschneck, unwrap and cut open.