In a medium bowl, combine the fish sauce, soy sauce, toasted sesame oil, sugar, and pepper; stir until the sugar dissolves. Add the beef and turn to coat well. Set aside for 30 minutes or cover and refrigerate for up to one day.
Heat an oiled grill or grill pan until very hot. Remove the meat from the marinade and let the excess drip off. Grill the beef until nicely browned on one side, 4 to 5 min. Turn and cook until browned on the other side and done to your liking, another 4 to 5 min. for medium rare. Transfer to a cutting board and set aside to cool to room temperature.
While the steak cools, toast the sesame seeds in a small dry frying pan over medium heat, shaking the pan and stirring until they’re lightly browned and fragrant, about 3 min. Spread the seeds on a small plate to cool.
Slice the steak across the grain into 4- to 6-inch-long, slender (about 1/8- to 1/4-inch-thick) strips. You should have at least 16 strips. Sprinkle on the toasted sesame seeds, toss to distribute them evenly, transfer to a plate, cover, and refrigerate until ready to roll. Use in Summer Rolls.