1.Combine stock, ginger and chilli in a medium saucepan; bring to the boil. Reduce heat; simmer, uncovered, for 5 minutes.
2.Add juice, sauce and beef to pan; return to the boil. Add pak choy; cook for 1 minute or until pak choy is wilted. Remove from heat.
3.Meanwhile, place noodles in a small heatproof bowl, cover with boiling water; stand until tender, drain.
4.Place noodles in a medium serving bowl; ladle over soup, top with onion and herbs.
Replace the baby pak choy with your favourite Asian green. For a little extra heat, slice a small red thai (serrano) chilli and add it to the soup with the herbs. If you prefer chicken, replace the beef stock with chicken stock and the beef with sliced chicken breast.