In a large bowl, combine the sliced cucumbers, tomatoes, red onion, and chopped parsley.
In a small bowl, whisk together the lemon juice, olive oil, salt, and black pepper to make the dressing.
Pour the dressing over the vegetables and toss to coat evenly.
Cover the bowl with plastic wrap and refrigerate for at least 30 minutes to allow the flavors to meld together.
Before serving, give the salad a quick toss to redistribute the dressing and ensure that all the vegetables are evenly coated.
Serve the cucumber tomato onion salad chilled.