Marinade the beef with half the fish sauce and lemongrass while you get everything else ready. You can even do this hours ahead or the night before.
Dipping sauce
Mix everything together and keep aside. Can be made a day earlier and kept, covered, in the fridge. Will last 2 days in the fridge.
Cook the beef
Heat the oil in a large frying pan over medium high heat and sauté the onions for a minute.
Add the garlic and lemongrass and continue for 30 seconds.
Add the beef strips, turn the heat up and mix thoroughly, stirring vigorously, for 30 seconds.
Add the rest of the fish sauce and continue cooking on high heat for 1 minute for pink in the middle or as done as you prefer.
Add the pepper and water if using, stir, turn heat off.
Check for seasoning, add salt if necessary. Leave aside while you assemble the salad in individual portions or take it all to the table for diners to help themselves to.
Assembly in order
Rice noodle bed
Bean sprouts and rest of the fresh stuff on and all around, see picture.
Beef
Spring onions
Peanuts
Drizzle some dipping sauce all over
Leave the sauce on the side for actual dipping too.
Notes
For a stronger flavour, marinate the beef for a couple of hours or even the night before.