Combine the tomatoes, cucumber, and red onion in a large mixing bowl with enough room for gentle tossing.
In a small bowl, whisk together the olive oil, red wine vinegar, Italian seasoning, kosher salt, and black pepper for 20 to 30 seconds, until the dressing looks slightly blended and the herbs are evenly suspended.
Taste the dressing and adjust with a small pinch of extra salt or pepper if needed.
Pour the dressing over the vegetables and toss gently with a wide spoon, lifting from the bottom so the tomatoes do not break down too quickly.
Serve right away for the crispest texture, or refrigerate leftovers in an airtight container for up to four days.
