Boil the noodles in salted water until just tender (about 3–4 minutes). Drain and rinse under cold, running water in a colander. You can cook these up to 3 hours ahead of time.
Measure 75ml water into the saucepan and add the fish sauce and soy sauce. Chop the spring onions and add to the liquid. Five minutes before serving, bring to the boil. Continue boiling