Using a 2½-inch round cutter, cut 16 rounds from bread slices. Place on a rimmed baking sheet. Bake until lightly toasted, approximately 5 minutes.
In a small bowl, stir together sour cream, green onion, capers, dill, lemon zest, lemon juice, horseradish, salt, and pepper.
To assemble: Place 1 cucumber slice in the center of each bread round. Top each with 1 teaspoon sour cream mixture. Shape smoked salmon into rosettes on top of sour cream. Garnish with fresh dill, if desired. Refrigerate until ready to serve.
Recipe by Southern Lady Magazine at https://southernladymagazine.com/smoked-salmon-canapes-recipe/