Chop the tomatoes and cucumber into bite-sized pieces and place them in a large mixing bowl.
Thinly slice the red onion and add it to the bowl with the other vegetables.
In a small bowl, whisk together the olive oil, red wine vinegar, Dijon mustard, minced garlic, salt, and pepper until well combined.
Pour the dressing over the vegetables in the large bowl.
Add the chopped parsley to the bowl.
Gently toss everything together until all vegetables are evenly coated with the dressing.
Let the salad sit at room temperature for 10 minutes to allow the flavors to meld.
Toss once more before serving.