Remove the ends of the cucumbers and cut lengthwise into halves. Using a teaspoon, scrape the pulp and seeds from the centre of the cucumbers and set it aside. Cut the cucumbers into 5cm batons.
Put the cucumber pulp into a sieve and, using the back of a spoon, force the juice from the pulp into a small bowl. You only need around one tablespoon, which is about one cucumber.
Add the remaining dressing ingredients to the juice and stir well.
to serve
Place spinach leaves, cucumber, onion, and dressing into a dish and mix well.