Cook the noodles according to package directions, using only half of the seasoning packet. To the broth, add the juice of 1 lime, a tablespoon of sugar, 2 teaspoons of fish sauce, and a good pinch of pepper flakes (if desired). Transfer to a serving bowl then add 1 to 2 ounces thinly sliced flank steak (it’ll cook in the broth), a handful of bean sprouts, and some mixed herbs (basil, cilantro, mint, as desired).