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 marinating 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 marinated 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 with the black stir fry sauce until they have been warmed through.
Divide the chow mein between 4 serving bowls, then drizzle a little sesame oil over each. Serve immediately.
For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home and view the full Food safety at home video.