Boil and mash potatoes. Add salt and cornstarch to thicken. Mix the onion with the beef, add salt and pepper and saute. Strain the juice from the saute. Form potatoes into balls. Stuff potatoes with meat so the potato is half full. Wrap the dumplings and brush them with egg yolk. Sprinkle non-flavored bread crumbs over the dumplings and bake them in a preheated 480 degree oven for 15 minutes or until they brown.