Put the lemon grass, garlic, chilli, shallot, half the sugar and 2 tablespoons fish sauce in a blender and whizz to form a chunky paste.
Rub the mixture all over the beef and thread the meat on to skewers. Drizzle over the oil. Heat a large griddle pan until smoking, then cook the skewers for 1–2 minutes on each side until charred.
Meanwhile, cook the noodles according to the pack instructions, cool under cold running water and drain. Stir together the remaining sugar and fish sauce with the lime juice. Toss through the noodles along with the lettuce, carrots and mint leaves and serve with the skewers.