Place 1 small thinly sliced red onion in small bowl and cover with cold water. Let stand while you prepare the rest of the ingredients, about 10 minutes.
Place 1 ½ pounds sliced heirloom tomatoes in a large bowl. Add ¼ cup coarsely chopped fresh herbs, 2 tablespoons extra-virgin olive oil, 1 tablespoon red wine vinegar, ½ teaspoon kosher salt, and ¼ teaspoon freshly ground black pepper. Drain the onions and add to the bowl. Toss to combine and let stand for at least 10 or up to 30 minutes before serving. Taste and season with more kosher salt, black pepper, vinegar, or olive oil as needed.
