Cut the tomatoes into ½-inch thick slices and arrange in a single layer in a large, shallow dish. Measure the oil, vinegar, onion, garlic, parsley, basil, salt, and pepper in a jar with a tight-fitting lid. Shake vigorously to combine and evenly pour over the tomatoes in the dish. Cover and marinate for 2 to 3 hours on the counter, or 12 to 24 hours in the refrigerator. Bring chilled tomatoes to room temperature before serving. Transfer tomatoes to serving dishes, spoon juices over the tops, and garnish with additional chopped fresh herbs, if desired.