1/2 cup fish sauce
1 tablespoon packed lime zest
1/3 cup fresh lime juice (from about 3 limes)
2 tablespoons dark brown sugar
2 garlic cloves, grated or minced
1 large jalapeño, seeds and veins removed if desired, minced
1 flank steak, about 1 1/2 pounds
1 small seedless English cucumber, thinly sliced
1 large bunch of radishes, thinly sliced
4 scallions, thinly sliced
1 teaspoon vegetable, peanut or olive oil (optional)
Cooked rice noodles or rice, for serving (optional)
Sesame seeds or crushed roasted peanuts, for serving (optional)
Fresh mint leaves or cilantro, for serving (optional)
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.
Light the grill or heat the broiler to high. Pat steak dry with paper towels.
If grilling, cook until done to taste, about 3 minutes per side for rare. If broiling, heat a 10-inch skillet over high heat for 5 minutes. Add oil to pan and sear the meat for 2 minutes. Flip meat and immediately transfer pan directly to top shelf of oven and broil for 2 to 3 minutes for rare, or longer for more well-done meat. Transfer steak to a cutting board and let rest for 10 minutes covered in foil.
Thinly slice steak and serve over cooked rice noodles or rice if desired, and top with the cucumber salad. Garnish with sesame seeds and plenty of herbs if using and serve with remaining marinade as a sauce.