In a small bowl, whisk together sour cream, horseradish, lemon juice, salt, and pepper.
Cut each pumpernickel slice into 2 (2½×2-inch) rectangles. Spread a thin layer of sour cream mixture onto each rectangle.
Using a sharp knife, cut smoked salmon into 2-inch strips. Shingle salmon pieces on top of sour cream mixture.
Garnish with radish and parsley, if desired. Serve immediately, or cover with damp paper towels, place in a covered container, and refrigerate for a few hours until serving time.
Recipe by TeaTime Magazine at https://teatimemagazine.com/smoked-salmon-canapes/