As far as a day in advance, cut the onion into bite-size squares. Soak them in a bowl of water in the refrigerator for up to a day, changing the water a few times in between.

When you're ready to make the salad, drain and dry the onions with paper towels.

Chop the cucumbers and tomatoes into bite size pieces and add them into a bowl along with the onions, parsley, lemon juice and salt.

Toss to distribute everything evenly and serve immediately.