1 small or 1/2 medium red onion, thinly sliced into half-moons
1 1/2 pounds heirloom tomatoes (about 3 medium tomatoes)
1/4 cup packed fresh herbs, such as basil, cilantro, dill, parsley, or a combination, coarsely chopped
2 tablespoons extra-virgin olive oil
1 tablespoon red wine vinegar
1/2 teaspoon kosher salt
1/4 teaspoon freshly ground black pepper
Place the onion in small bowl and cover with cold water. Let stand while you prepare the rest of the ingredients, about 10 minutes.
Core the tomatoes and slice into 1/2-inch-thick half-moons. Place in a large bowl. Add the herbs, oil, vinegar, salt, and 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 salt, pepper, vinegar, or oil as needed.