Bring stock, garlic, spring onion and ginger to the simmer over medium-high heat in a large saucepan. Add chicken, reduce heat to low and poach until just cooked through (10-12 minutes). Remove chicken, set aside until cool enough to handle, then coarsely shred and set aside. Increase heat to medium-high and bring stock to the simmer. Add corn and cook for 1 minute. Add noodles and gai lan, bring to the simmer and stir until gai lan is bright green (1-2 minutes). While stirring, add egg in a thin steady stream, return to the simmer and cook until just set (1 minute). Add chicken, season to taste and ladle into serving bowls.
Meanwhile, stir soy sauce, chilli, sesame oil and caster sugar in a small bowl to combine. Serve with hot soup topped with coriander.