In a medium bowl, combine cucumber, red onion, lemon peel, lemon juice, salt, and pepper; toss to coat.
Spread butter onto one side of each bread slice. Top each with cucumber mixture. Cut each sandwich diagonally into quarters.
In a small bowl, combine sour cream and chives. Top each sandwich with a dollop of the sour cream mixture. Makes 24 appetizers.
Cover and chill sandwiches up to 2 hours before serving.