Soak the onion in the lemon juice for 1 hour. Soak and cook the noodles and keep warm.
Place the warm noodles, onion rings, herbs, lettuce, shredded beef, chilli, garlic, spring onion, fish sauce and lemon wedges on the table in bowls.
Place the boiling stock on the table with a ladle for serving.
Place a layer of noodles, onion rings, herbs, shredded lettuce, shredded beef, chilli, garlic and spring onion in a warmed individual soup bowl. Pour boiling stock over the ingredients, releasing the fragrances and cooking the beef.
Season with fish sauce and lemon juice to taste.