Core the tomatoes, cut into wedges. Then cut each wedge in half crosswise. Place the tomatoes in a large bowl.
Cut off the ends of the cucumbers. Using a vegetable peeler, make long stripes going top-down. Slice into 1/4″ thick slices slightly on the diagonal. Transfer to the bowl with the tomatoes.
Cut the avocado in 4 quarters lengthwise and remove the pit. Gently remove the skin, then slice and add to tomatoes and cucumbers, along with the diced onion.
To prepare the dressing, throw olive oil, red wine vinegar, dijon mustard, dried oregano, garlic, salt, and black pepper into a small bowl and whisk together vigorously, until well blended.
Pour the dressing over the vegetables and toss gently using both hands. You can serve immediately or cover the bowl with a plastic wrap and let sit at room temperature until ready to serve. This salad only gets better with time.