Cut the beef into thin slices. Remove tops from the lemongrass stalks and discard. Finely slice the white and light green stalks. Combine 2 tbsp of fish sauce, sugar and half the lemongrass in a bowl. Add beef slices to bowl, stir and leave to marinate for 10-15 minutes.
In a pot, boil enough water to cover the noodles. Turn off heat and immerse rice noodles for 3-7 minutes until noodles are soft, cooked through but still firm, not mushy. Rinse the noodles under cold water for 30 seconds, then place them in cold water in the fridge until you are ready to assemble the dish.
For the nuoc cham dressing, pound chilli and garlic in a mortar with a pestle to form a paste; add the palm sugar and pound together. Stir in other liquids slowly until they are completely incorporated. Check the balance of seasoning: it should be salty, sweet and sour.
Heat the oil in a wok over a very high heat and add the eschalot, garlic and remaining lemongrass. Stir-fry briefly, then throw in the beef, allowing it to char and develop a smoky flavour. Add remaining fish sauce, then remove from the heat.
Drain the noodles well, share over four bowls and top with bean sprouts, cucumber and most of the herbs. Add beef to the bowls and dress with nuoc cham. Garnish with remaining herbs, peanuts and fried shallots.