For the marinade, roughly chop the lemongrass, ginger, chilli and the stalks of the coriander, reserving the leaves for later. Add to a food processor and pulse until very finely chopped. Add the dark soy sauce and a dash of sesame oil, to taste. Slice the steak into thin strips, and add to the marinade. Leave to one side.
Place the noodles into a heatproof bowl and pour over enough boiling water from the kettle to cover. Leave for 4-5 minutes, or until softened. Drain the noodles and rinse with cold water to prevent further cooking. Leave to one side. In a dry hot pan, fry the sesame seeds for 2-3 minutes, or until browned all over. Set aside.
Heat a large frying pan over a high heat and add a little oil. Remove the strips of steak from the marinade, reserving it for later. Fry the steak strips in the hot pan for one minute on either side, adding the reserved marinade to the pan for the final minute of cooking. Set aside once cooked and sprinkle with chopped coriander.
In a large bowl, toss together the noodles, grated carrot and the oyster sauce, making sure that the noodles are evenly coated with the oyster sauce. Sprinkle with the toasted sesame seeds.
Add a generous spoonful of noodles into each of the lettuce leaves, top with a few strips of steak in each, garnish with the coriander leaves and serve.