Peel onion and chop very finely. Mix well the sausage fat and onion. Spread pumpernickel with butter on one side each. Cover one slice of pumpernickel with cheese and fold a slice of bread on top.
Cover another pumpernickel disc with salmon (several layers) and also fold a disc on top. Spread the grease on another slice and cover with the last slice. Cut each pumpernickel slice with a sharp knife into 9 equally sized cubes.
Fill mustard, mayonnaise and horseradish into one disposable piping bag each, cut off the tip with sharp scissors. Garnish the diced meat with mayonnaise, diced salmon with horseradish and diced cheese with mustard dots.
Arrange on a plate.