Peel and chop the onion. Knead well with ground pork
Spread pumpernickel with butter. Cover one slice with the cheese slice and one thickly with salmon. Spread one slice with ground pork. Put the remaining slices of bread on top. Cut with a sharp knife into 9 equally sized cubes
Garnish diced meat with mayonnaise, diced salmon with horseradish and diced cheese with mustard dots. Dress everything