Peel the cucumber and cut it in half lengthwise. Remove the seeds from each half with a spoon and discard. Slice the cucumbers very thin on a slight angle and place in a bowl. Slice the onion very thin, fine dice the red pepper and chop the cilantro. Place in the bowl with the cucumbers. Add the oil, vinegar, sugar and salt to the cucumbers. Cover and cool in the refrigerator for an hour so the cucumbers can absorb the flavors as they cool.
* you may also puree the dressing in a blender or food processor.
* add fresh lemon or lime juice right before serving.