If marinating the beef (optional, but does add flavor), mix together the marinade ingredients - soy sauce, fish sauce and pepper - and add the beef. Marinade for around 2 hours, if possible, or at least 30 minutes. Refrigerate if more than 30 minutes then remove from fridge 30 minutes before cooking to come to room temperature.
Gently toast the rice in a dry skillet/frying pan until it is is gently brown. Grind it up with a pestle and mortar so that it's relatively fine. This can be done ahead of time and stored in a sealed container at room temperature.
Thinly slice the shallots and scallions and roughly chop the mint and cilantro.
Grill the steak over a medium high heat on both sides. Timing depends on thickness, heat etc but you are looking for medium rare to medium at most (I also like it with rare), so still a bit of bounce when you press the middle. For a medium thickness steak, that's is likely around 6 - 8 minutes in total but will depend on many factors so check as you go. Once cooked, remove from the grill and let it rest a couple minutes before slicing thinly - be sure to save the juices as well.
Add the beef along with any juices from it to a mixing bowl with the herbs, shallots and scallions. Add the fish sauce, lime juice, ground toasted rice and chili flakes then toss everything in the bowl.