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.
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.
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.