Season steak: Trim lemongrass ends, then remove the fibrous outer layers (about 2–3 layers). Thinly slice the tender inner core; you should have about ¼ cup. Place a cloth towel under the mortar to keep it stable; add 1 tablespoon sliced lemongrass and the peppercorns to the mortar. Loosely pound to break up while also scraping the sides, 10 seconds. Set aside. Place steak on a plate and pour soy sauce over to coat both sides. Rub the lemongrass-peppercorn paste on both sides of the steak, pressing to adhere. Let rest at room temperature, 10 minutes.
Grill steak: Preheat charcoal grill according to grill directions to medium heat, about 30 minutes. Lightly brush preheated grill with oil, then brush the steak with oil. Grill the steak for 8-10 minutes, flipping once halfway through, until lightly charred on both sides. Meanwhile, prep the salad: Trim, peel, and thinly slice a shallot with the grain. Set aside, along with mint leaves, cilantro, and remaining sliced lemongrass.
Dressing: In a pan, add beef stock, fish sauce, lime juice, sugar, and chile powder. Heat sauce over medium heat just until heated and bubbly at the edges, 2 minutes. Turn off heat and set aside.
Assembly: Remove steak from grill and let rest, 10 minutes. To serve, cut the steak against the grain in thin slices; add to the pan with the warm dressing, along with any juices from the steak. Place the additional salad ingredients into the pan, along with dried rice powder. Stir to combine and coat ingredients with the dressing. Spoon onto a plate, sprinkle with additional toasted rice powder, and serve.