In a small bowl, whisk together the fish sauce, lime juice, brown sugar, scallions, garlic, and jalapeño. (If you think your fish sauce is very salty, start with 1 teaspoon; you can add more at the end.)
Arrange the tomato slices on a plate. Spoon the dressing over the tomatoes. Let stand for 10 minutes to allow the tomatoes time to release their juices. Sprinkle with the basil and cilantro; serve.