Using a 2-inch round cutter, cut 12 rounds from frozen bread slices, discarding scraps. Cover bread rounds with damp paper towels to prevent drying out while letting thaw.
Using a small hand-held mandoline or a sharp knife, cut 48 thin, uniform round slices from cucumber. Lay slices in a single layer on paper towels to absorb excess moisture.
In a small bowl, stir together mayonnaise, watercress, lemon zest, lemon juice, salt, and pepper until combined to make an aïoli. Spread aïoli in an even layer onto bread rounds.
Stack 2 cucumber slices together. Using a sharp knife, cut stacked slices in half to form half moons. Repeat with 2 more cucumber slices.
Referring to how-to photos below, on a bread round, place 1 stack of halved cucumber slices on top of aïoli. Place another stack of halved slices at a 90-degree overlapping angle to the first stack. Place a third stack at a 90-degree overlapping angle to the second stack. Place the fourth stack at a 90-degree overlapping angle to the third stack and underlapping the first stack.
Garnish with a watercress leaf, if desired.
Notes
Make-Ahead Tip: Aïoli can be made up to a day in advance, stored in an airtight container, and refrigerated to allow flavors to meld. Canapés can be made up to an hour before serving, covered with damp paper towels, placed in an airtight container, and refrigerated.
Recipe by TeaTime Magazine at https://teatimemagazine.com/cucumber-watercress-canapes/