In a small mixing bowl, use your hands to toss together flank steak, lime juice, fish sauce, crushed garlic, and crushed red pepper. Cover the bowl with plastic wrap and refrigerate up to 1 hour.
Make the dressing in a small mixing bowl by whisking together lime juice, fish sauce, soy sauce, sugar, and crushed red pepper flakes. Refrigerate until ready to serve.
Once the beef has marinated, heat a heavy skillet or grill pan over high heat. Cook the beef, 1 to 2 minutes per side and then transfer to a bowl. Working in batches may make this easier and it cooks so quickly it shouldn't take more than 10 minutes to cook a whole pound of beef.
Compile the salad on a bed of romaine lettuce topped with tomatoes, cucumbers, fresh cilantro, red onion, and the beef. I like to make mine in layers so that the toppings are dispersed throughout the salad. Pour the dressing generously over the salad before serving.
If you are in a hurry, you can skip marinating the beef. Marinating it for an hour just gives it a little more time to soak up the flavor.