Using a food processor or handheld mandoline, thinly slice the cucumbers and place them in a large bowl. Sprinkle with salt and toss well to coat evenly.

Place a plate on top of the salted cucumbers to weigh them down, and let sit at room temperature for at least 1 hour, until the cucumbers have released much of their water and wilted.

Drain the cucumbers, rinse under cold water, and drain again. Add the cucumbers to a larfge bow, and add the sliced onion to the cucumbers.

Combine the vinegar and 1 cup cold water and pour over the cucumbers. If using sugar, it is best to dissolve it in a little hot water and then add that mixture to the cucumbers.

Pack the cucumbers and onions into a 1-quart glass jar or other non reactive container and refrigerate. The cucumbers can be eaten right away, but they are best after they have sat for a day or two. They will keep about two weeks in the refrigerator.