Wash the cucumbers and tomatoes thoroughly.
Peel the cucumbers if desired, then dice them into bite-sized pieces.
Chop the tomatoes into similar-sized pieces as the cucumbers.
Thinly slice the red onion and add it to a large mixing bowl.
Add the diced cucumbers and chopped tomatoes to the bowl with the onions.
Chop the fresh parsley and sprinkle it over the vegetables.
In a small bowl, whisk together olive oil and red wine vinegar.
Pour the dressing over the salad ingredients in the bowl.
Season with salt and pepper to taste, mixing gently to combine.
Let the salad sit for about 10 minutes to allow the flavors to meld.