Thinly slice the steak and put in a bowl. Place shallots, garlic, lemongrass and chili in a large mortar and use the pestle to grind the mixture into a fine paste. Add the sesame seeds and crush with the pestle. Spoon the paste and the fish sauce over the meat, cover and refrigerate for 1 hour.
Soak 12 bamboo skewers in water for 1 hour to prevent them from burning during cooking.
Thread the meat onto the skewers and cook on a hot chargrill or barbecue grill for 2-3 minutes on each side. Meanwhile split the baguette in half lenghtways.
Place the lettuce, cucumber, coriander sprigs and extra chili inside the baguettes.
Place two fo the cooked skewers inside each baguette. Hold onto the baguette and gently pull out the skewers
Note
Marinating the beef in the mixture give it a lot of flavor and spice. Make sure you marinate it for at least an hour.