Peel the cucumber and slice into 0.6-cm (¼-inch) rounds.
Whisk the sour cream, lemon juice, dill and pepper, then spread onto the bread slices.
Arrange 2 layers of cucumber slices onto 1 bread slice, and top with the remaining slice.
Cut into quarters or strips and garnish with a cucumber slice on a toothpick, dill and a sprinkle of pepper.
Serve immediately.