Peel away the skin of the cucumber in alternate stripes lengthwise. Halve and then cut into slices and put in a salad bowl.
Add the tomatoes, celery, olives, onion, lettuce and coriander to the bowl. Season with Sea salt and freshly ground black pepper and mix thoroughly.
Mash the garlic until it is a smooth paste with a pinch of salt and then mix into the vinegar, mixing well until smooth. Whisk in the oil. Pour over the salad and mix gently to combine. Crumble the feta cheese and oregano over the top before serving.