In a small bowl, combine the fish sauce, lime zest, lime juice, brown sugar, garlic, and jalapeño. Pour 1/2 of the mixture over the flank steak and let marinate at room temperature for at least 30 minutes (or refrigerate for up to 24 hours). Let meat come to room temperature before cooking if necessary.
Combine the cucumber, radishes, and scallions. Pour in just enough of the marinade sauce to coat. Save some marinade to serve as a sauce.