Heat a ridged griddle pan until very hot. Brush the steak with the oil and season liberally with salt and pepper. Add the steak to the pan and cook over a high heat for 2 minutes on each side, making sure that you sear all over. Leave to rest for 5 minutes, then cut into thin slices. The steak should be rare.
Meanwhile, make the dressing. Place the fish sauce, lime juice and sugar in a bowl and stir in the chilli until the sugar has dissolved.
Split the rolls in half and fill with the herbs, beef slices and any juices. Pour the dressing carefully over and serve.