Make the beef marinade: mix together the nuoc mam, garlic, lemongrass and sugar. Cut the beef in thin slices, mix with the marinade, and leave for half an hour (longer if you can).
On a medium stove, cook the beef until it's done. Cover, and take away from the stove.
Pour boiling water on the bun noodles, and mix them with chopsticks until they turn opaque white and are no longer brittle. Drain the noodles and leave them to cool.
Pound the peanuts into small pieces with a mortar and pestle.
Put the bowls together: put half the noodles in them, followed by the beef, followed by the salad torn into small bits, followed by the soybeans, mint, coriander, and crushed peanuts.
Add about 3 tablespoons of nuoc cham, or to taste. Mix well, and eat.
Recipe by Aliette de Bodard at https://www.aliettedebodard.com/recipes/bo-bun-beef-rice-vermicelli/