Using a long, serrated bread knife in a gentle sawing motion, trim and discard crusts from each bread slice to make a 3 ¼x3-inch rectangle. Cut each bread rectangle in half to make 2 (3 ¼x1 ½-inch) rectangles.
In a small bowl, stir together mayonnaise, feta cheese, watercress, dill, lemon juice, salt, and pepper. Spread a layer of mayonnaise mixture onto bread rectangles.
Using a sharp knife, cut cucumber in half crosswise and each in half lengthwise. Using a mandolin, cut very thin lengthwise slices from cucumber halves. Cut slices crosswise into 1 ½-inch pieces. Blot cucumber pieces dry on paper towels.
Beginning at a short end of each bread rectangle, arrange 4 cucumber slices in a shingled, evenly spaced, parallel fashion atop mayonnaise layer. Serve immediately, or cover with damp paper towels, place in a covered container, and refrigerate for up to an hour before serving.
Just before serving, garnish each canapé with a watercress sprig, if desired.
Recipe by TeaTime Magazine at https://teatimemagazine.com/cucumber-feta-canapes/