In a small bowl, stir together crème fraîche, cream cheese, onion, lemon zest, lemon juice, salt, and pepper to combine.
Using a sharp paring knife, cut 12 thin slices from cucumber. (Blot cucumber slices on paper towels before assembling sandwiches.)
Using a serrated bread knife in a gentle sawing motion, trim and discard crusts from bread slices, creating perfect 3-inch squares. Stack 2 bread squares, cut in half horizontally, then cut in half again vertically, creating 4 perfect (1½-inch) squares for a total of 8. Repeat with remaining bread squares.
Spread a layer of crème fraîche mixture onto bread squares. Place watercress on 12 bread squares. Place a cucumber slice on top of watercress. Top each with a remaining bread square, crème fraîche side down. Serve immediately, or cover with damp paper towels, place in a covered container, and refrigerate for up to 1 hour before serving.
Garnish each with a watercress sprig, if desired.
Notes
Crème fraîche mixture can be made a day in advance and stored in a covered container in the refrigerator.
Recipe by TeaTime Magazine at https://teatimemagazine.com/cucumber-creme-fraiche-tea-sandwiches/