Finely grate 1 t. lemon zest; squeeze 3 T. juice from lemon over a medium bowl.Add zest, mayonnaise, and pepper to bowl; stir well.Let stand for about 5 minutes.Add salmon to bowl; stir until well blended.
Layer cucumber evenly over 4 bread slices; top each with about 1/2 cup salmon mixture.Top with remaining 4 bread slices.