In a mixing bowl, combine the soy sauce, sesame oil, and sugar. Whisk until the sugar is dissolved. Place the beef filet in the marinade, ensuring it is well coated. Let it marinate for at least 30 minutes at room temperature, or up to 2 hours in the refrigerator for more flavor.
Heat the neutral oil in a skillet over medium-high heat. Once hot, remove the beef from the marinade (reserve the marinade for later). Sear the beef for about 1-2 minutes on each side, or until a nice crust forms. The interior should remain rare to medium-rare. Remove the beef from the skillet and let it rest for a few minutes before slicing.
While the beef is resting, julienne the carrot, slice the green onion, and thinly slice the red chilies. Wash and dry the napa cabbage leaves, which will serve as the base for your rolls.
Slice the beef filet into thin strips. Take a napa cabbage leaf and lay it flat on a clean surface. Place a few pieces of beef in the center, followed by a sprinkle of carrot, green onion, and red chilies. Add a few cilantro leaves for freshness. Carefully roll the cabbage leaf around the filling, tucking in the sides as you go to create a neat roll.
Arrange the rolls on a serving platter. Drizzle with the reserved marinade and sprinkle with toasted sesame seeds. Garnish with additional cilantro for a pop of color. Serve immediately and enjoy!