Slice your beef into very thin slices, about 3mm or so, against the grain. To make the marinade, finely chop ½ a lemongrass and ½ a chilli. Add them to a pestle and mortar along with 2 garlic cloves and ½ inch ginger. Crush to form a coarse paste. Add to the beef along with 1 tbsp each of fish sauce, oyster sauce, sugar and oil. Stir to coat the beef, then cover in cling film and leave to marinate in the fridge for 1-24 hours.
Prepare the rest of your ingredients. If frying your own shallots, thinly slice, and fry in a pan with 3 tbsp oil on medium high heat for 5 minutes or so until crispy. Remove them and leave to drain on some kitchen paper. You can save the oil to use for the beef.
Pickle the cucumber. Slice it into thin slices, then add to a bowl and coat with the sugar and salt. Leave for 15 minutes, then rinse with cold water.
While the cucumber sits, roughly chop your cabbage into strips. Finely chop the other ½ lemongrass and fry in a pan until crispy. Then remove it and set aside.
When ready to eat, cook your noodles according to the package instructions. For me, this was submerging in boiling water for 3 minutes, then draining. Once drained, rinse well with cold water.
Heat a little oil on medium high, then add the beef slices, spreading out to make sure they don't overlap. Cook, stirring, until well charred - don't worry about overcooking, you want the crispy texture and blackened bits.
Add your noodles to a bowl, and toss with 3 or 4 tbsp of the noodle sauce. Top with the beef and your prepared vegetbales. Sprinkle on the fried lemongrass and shallots, and tear over some mint and coriander, and chopped fresh chilli if you like.