First, cut off the ends of the cucumbers. Then slice into 1/4" thick slices slightly on the diagonal. Add to the large bowl.
Next cut the tomatoes in half, then cut each half lengthwise into 3 wedges. Then cut each wedge in half crosswise. Transfer to the bowl with the cucumbers.
Cut the avocado in half lengthwise and remove the pit by twisting the halves gently. Next remove the skin, then slice into 1" thick slices and add to tomatoes and cucumbers.
Place the cucumbers, tomatoes, red onion, and avocado in a large bowl.
In a small bowl, whisk together the olive oil, red wine vinegar, Italian seasoning, sea salt, pepper and chopped parsley.
Pour the dressing over the vegetables and toss. Taste the salad and add more salt and pepper, if necessary.