Put the ginger, soy, mirin, garlic, pineapple, chilli flakes, sugar and 1 tsp of the sesame oil in a food processor and blend until fine. Pour the marinade into a bowl, add the meat, mix well and leave to sit while you prepare the onion.
Heat the remaining sesame oil in a large wok or frying pan until very hot. Add the onion and stir-fry for a few mins. Add the beef and the marinade, stirring constantly until it’s cooked through, about 5 mins. Sprinkle with the sesame seeds and serve with rice and chopped spring onions.
