Russian meat balls (bitochki)

1 lb Ground beef
      1 lb Ground veal
    1/2 c  Chopped onion
    1/4 c  Rendered kidney fat
      2 sl Break, soaked in milk,
           -squeezed dry
      2 ts Salt
           Ground pepper
           Fine bread crumbs
           butter or beef fat
      2 c  Sour cream
    1/2 lb Sliced mushrooms, sauted
 
  Cook onion in rendered kidney fat until wilted. Mix beef, veal, onion,
  bread, salt and little pepper. Knead well and chill. Wet hands and form
  mixture into balls the size of gold balls. Roll in crumbs and fry in butter
  or beef fat until brown all over. Remove and keep warm. Add sour cream and
  mushrooms to pan. Heat. Pour sauce over meat.