Prepare Ahead

Step 1 can be completed 1 hour in advance, and the cucumber slices kept covered in the refrigerator.

Instructions

1. 1

   Using a sharp knife, carefully slice the cucumber into thin slices, no thicker than 3mm (1/8in) for each slice. Place the slices in a colander or sieve and sprinkle with the salt. Leave the colander to sit over a plate for 20–30 minutes to allow the excess water to drain out of the cucumber slices. When ready to use, sprinkle the slices with the vinegar.

   
   

2. 2

   Spread each slice of bread with the softened butter. Place the cucumber in an even layer over four of the bread slices. Grind with some black pepper then top with the remaining bread.

   
   

3. 3

   Cut off the crusts, then cut each sandwich into squares, triangles or fingers. Arrange the sandwiches on a serving plate and sprinkle with watercress. Serve at once.