In large bowl whisk together all dressing ingredients.
Add cucumbers, tomatoes and celery (also onion if using). Gently stir to combine.
Serve immediately.
*If you want to prepare ahead of time, make dressing and refrigerate in closed container. Prepare the rest of the ingredients and place in 3 or 4 separate zip lock bags. Place in refrigerator until ready to use. Drain any excess liquid from the bags then combine with dressing.