Mix together horseradish, mayo, and pepper in a small bowl. Set aside.
Cut circles out of the roast beef.Use the same cookie cutter to cut out 24 roast beef circles.
Cut circles out of the bread.Use the cookie cutter to cut out 12 bread circles out of the white bread. Cut 2 circles out of each slice of bread, avoiding the crust.
Assemble tea sandwiches.From the bottom up: bread, horseradish mayo, roast beef, horseradish mayo, bread, horseradish mayo, roast beef, horseradish mayo, bread, and watercress skewered on with a cocktail fork.Start with a bread circle, then spread on a thin layer of horseradish mayo, and top with 3 roast beef circles. Spread on a thin layer of horseradish layer, top with bread and repeat with the horseradish mayo, 3 roast beef circles, horseradish mayo, then bread. Place the watercress on top and keep it in place with a cocktail fork