Kosher or coarse salt and freshly ground pepperto taste
Combine the brown sugar, oil, fish sauce, lime juice, garlic and red pepper flakes in a large container or study zipper top bag. Remove two tablespoons of the marinade and set aside in a small bowl. Add the meat to the rest of the marinade, turn to coat well, and marinate in the fridge for 1 to 6 hours.
When you are ready to grill the meat, preheat the grill to medium high. While the grill is heating, make the salad: in a large shallow serving bowl, toss together the lettuce, cucumber, red pepper and scallions. Make the dressing: in a small container combine the rice vinegar, oil and salt and pepper. Shake to combine.
When the grill is hot, grill the steaks for about 4 minutes per side, watching carefully to see that the steaks aren’t burning due to the sugar in the marinade. After the outsides are nicely browned you may need to move the steak to a cooler part of the grill, or lower the heat on a gas grill, so that the inside cooks before the outside chars. 125°F is the internal temperature for medium rare, 135°F is the temperature for medium. Let the steaks sit on a cutting board for 5 minutes to re-absorb their juices and finish cooking before slicing across the grain.
Toss the salad with the dressing, and then lay the slices of steak across the salad and drizzle with the reserved marinade, either in the serving bowl, or you can let everyone serve themselves salad and add the meat to their own plates, giving the meat on their own plates a teaspoon of marinade.