Add cucumbers, tomatoes, and onion to a large bowl. (If you don’t like eating the skin of cucumbers, or if the skins are particularly bitter-tasting, feel free to completely peel them prior to slicing. Sometimes I will just run a peeler down the length of the cucumber a few times, removing about half the skin and creating a pretty striped effect.)In a medium bowl, stir together the remaining ingredients to create the dressing. Pour over the cucumber mixture and fold gently to combine. Place salad in a serving bowl, sprinkle with a bit more black pepper, and serve immediately.This can be prepared many hours in advance, or even the day prior to serving. Just store the cucumber mixture and dressing mixture in separate containers in the refrigerator, combining them just prior to serving.