Chop the tomatoes and cucumbers to about the same size. Cut the onion half into quarters then thinly slice each quarter. Add all the veggies to a large mixing bowl. If you are adding fresh parsley or other herbs, you can add them at this point as well!
Combine the olive oil, vinegar, lemon juice, Dijon, minced garlic, Italian seasoning, salt, and pepper. You can shake it in a mason jar with a lid, whisk it in a container, or mix it in a personal blender. We prefer to use a blender as it emulsifies it the best.