Whisk together the soy sauce, cornflour and sugar in a small mixing bowl until smooth.
Place the beef in a large mixing bowl and coat with the marinade, mixing well with your hands to coat evenly. As the meat is marinading blanche the egg noodles in a large saucepan of boiling, salted water for 3-4 minutes until softened. Drain and refresh in cold water, setting to one side until later.
Heat the groundnut oil in a large wok over a moderate heat until hot. Stir-fry the onion and spring onion, then add the marinaded beef. Cook for 2-3 minutes, stirring all the time. Drain the the noodles really well, then add to the wok and stir-fry for a further 1-2 minutes until they have been warmed through.
Divide the chow mein between 4 serving bowls, then drizzle a little sesame oil over each. Serve immediately.
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