Using a long, serrated bread knife, trim and discard crusts from bread slices. Cut each bread slice into 3 (3x1 ½-inch) rectangles.
Using a long sharp knife in a gentle sawing motion, cut cheese slices into 12 (approximately 3x1 ½- inch) rectangles.
Spread an even layer of Chive-Horseradish Mayonnaise onto each bread rectangle. Place a layer of watercress leaves over mayonnaise side of 12 bread rectangles. Add a cheese rectangle to each. Ruffle and fold roast beef slices to fit on top of cheese. Top each with a remaining bread rectangle, mayonnaise-side down.
Serve immediately, or cover with damp paper towels, place in a covered container, and refrigerate until ready to serve.
Recipe by TeaTime Magazine at https://teatimemagazine.com/roast-beef-cheddar-watercress-sandwiches/