Cook rice until tender, then drain and keep hot. Combine stock, soy sauce and mirin in a small bowl.
Heat oil in a wok or a non-stick frying pan and stir-fry spring onions for less than a minute. Add beef slices and turn immediately they change colour – a matter of seconds.
Add stock mixture and allow to bubble. Quickly add ginger, then remove pan from heat.
Pile rice into a heated bowl (preferably one with a lid), then add contents of pan. Cover for a moment before eating.