In a small bowl, combine mayonnaise, horseradish, and pepper, stirring well.
Using a serrated bread knife in a gentle sawing motion, trim and discard crusts from bread slices. Cut each slice in half, making 2 (3×1½-inch) rectangles.
Using a chef’s knife, cut 16 (3×1½-inch) rectangles from salmon.
Spread a layer of mayonnaise mixture onto bread rectangles. Top each of 8 bread rectangles with 2 salmon rectangles and with another bread rectangle, mayonnaise side down.
Serve immediately, or cover with damp paper towels, place in a covered container, and refrigerate until serving time.
Recipe by TeaTime Magazine at https://teatimemagazine.com/piquant-salmon-tea-sandwiches/