In a small bowl, whisk together crème fraîche, horseradish, and mint until mixture thickens.
Using a 3-inch square cutter, cut 6 shapes from frozen bread slices, discarding scraps. Return bread squares to freezer.
Using a mandoline, cut 12 paper-thin slices from cucumber lengths. Place on paper towels to let dry.
Using a sharp knife, cut 18 (3x½-inch) rectangles from smoked salmon slices, discarding scraps.
Spread an even layer of crème fraîche mixture onto frozen bread squares. Lay 2 slices cucumber side-by-side on crème fraîche side of each bread square, trimming cucumber to fit bread. Lay 3 smoked salmon rectangles over cucumber, with space between to show cucumber.
Using a sharp knife in a down-ward motion, cut canapé in half diagonally. Cover with damp paper towels, place in a covered container, and refrigerate until bread thaws and is ready to serve, up to an hour.
Recipe by TeaTime Magazine at https://teatimemagazine.com/salmon-cucumber-canapes-recipe/