1. In a large glass baking dish, mix the soy sauce with the pepper and 2 tablespoons of the lemongrass. Add the flank steak and turn to coat. Let stand at room temperature for 30 minutes.
2. Light a grill. Grill the flank steak over moderately high heat, turning once, until charred on the outside but still pink within, about 8 minutes. Transfer the steak to a carving board and let stand for 5 minutes. Cut the steak in half lengthwise. Slice the halves across the grain 1/4 inch thick.
3. In a large wok or nonreactive skillet, combine the lime juice with the fish sauce, crushed red pepper, sugar and the remaining 2 tablespoons of lemongrass. Cook over moderate heat until hot. Stir in the sliced steak along with 1 tablespoon of the marinade. Add the shallots, mint, cilantro and roasted rice powder, stirring to coat. Transfer the salad to plates and serve.
NOTE Rice powder is available at Asian markets but can also be made at home. In a skillet, toast raw glutinous (sticky) rice over low heat, stirring occasionally, for 30 to 45 minutes. Transfer to a mortar or spice grinder and let cool. Grind the rice to a powder.