Cut the cucumber in half remove the seeds with a spoon then 1/4 inch thick slices.
Half the cherry tomatoes and set aside.
Peel and slice the onions in cubes and separate the layers.
Place all the ingredients in a big bowl mix well.
Cover with a cling film and set in the refrigerator to chill.
When ready to serve in a small bowl add the ingredients except for the olive oil and whisk together.
Add the olive oil slowly while whisking the ingredients in the small bowl.
Drizzle this vinaigrette emulsion over the salad.
Add the feta and olives and toss lightly.