Pat the steak very dry and season with a pinch of salt. Dry meat is the difference between a real sear and gray steam, so do not skip this.
Heat the oil in a heavy skillet or grill pan over high heat until it just starts to smoke. Sear the steak 3 to 4 minutes per side for medium-rare, until a deep crust forms. Move it to a board and rest 8 to 10 minutes.
While the beef rests, stir the fish sauce, lime juice, sugar, and chili flakes together until the sugar dissolves. Taste. It should be sharp, salty, and a little hot.
Slice the rested steak thinly across the grain. Tip the slices and any juices into a bowl.
Add the shallots, scallions, and dressing and toss. Add the toasted rice powder and most of the herbs, toss once more, and pile onto a plate. Scatter the rest of the herbs on top and serve right away.