Kosher salt and freshly ground black pepper,to taste
Place the tomatoes and red onion in a large bowl. Drizzle with olive oil and red wine vinegar. Gently stir. Add the fresh mozzarella, if using. Garnish with fresh basil and season with salt and black pepper.
You can serve immediately, but if you have time, let the salad sit, tossing occasionally, until juicy and deeply flavored, at least 30 minutes and up to 1 hour.