Using a fork, whisk the cornflour, egg and soy sauce in a bowl until well combined. Add the chicken pieces and turn to coat on all sides.
Heat the sunflower oil in a large wok over high heat, add the chicken and stir-fry for 3 minutes or until crispy and light golden. Add the onion, garlic, ginger and capsicum and stir-fry for a further 3 minutes.
Add the tomatoes, tomato sauce, hoisin and vinegar and toss to combine, then immediately remove the wok from the heat.
Divide the steamed vegetables among serving bowls, top with the sweet and sour chicken and serve with finely sliced spring onion and red chilli, if using.