Cut the cucumber into very small cubes. Remove the seeds of the tomato and chop the flesh into small cubes. Allow them to drain their juices for a while.
Mix the olive oil with the lemon juice, salt and pepper to taste. In a large bowl, mix the yoghurt with the cucumbers, tomatoes, walnuts and flavour it with the dressing.
Stir and let the salad cool for 1 hour. Serve with chopped chives.