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Mix the steak, soy sauce, sugar, chilli sauce and vinegar together, and set aside for 10 mins.
Heat a wok with 1 tsp of the oil. Lift the steak from the marinade and sear, in batches, then remove. Add a bit more oil and fry the ginger until golden. Add the spring onions and pineapple, and return the steak to the pan. Stir to heat through for 1 min, then add any remaining marinade. Keep stirring until the marinade becomes thick and everything is hot. Serve sprinkled with coriander, and with rice and greens, if you like.