1In 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. If refrigerated, let meat rest on counter 15 to 20 minutes, to come to room temperature, before cooking.)
2Pat steak dry with paper towels. Preheat broiler or outdoor grill.
If grilling, grill steak on hot grill for about 3 to 4 minutes per side or done to taste.
If broiling, heat a 10-inch oven-proof cast iron skillet over high heat for 5 minutes. Add steak and sear meat in hot skillet for 2 minutes. Turn steak and transfer skillet to oven and broil for 3 minutes per side for rare (or longer for medium or well-done steak).
3Transfer steak to cutting board, cover with foil, and let rest 10 minutes.
4While steak is resting, prepare Cucumber Salad. Combine the cucumber, radishes, scallions, peanut oil and rice vinegar, and pour over salad. Add just enough of the marinade sauce to coat vegetables, and toss gently.
5Thinly slice steak and arrange beef slices over cooked rice or cooked noodles. Sprinkle sesame seeds over steak slices. Top with cucumber salad, and garnish salad with fresh mint and/or cilantro leaves. Serve immediately.