Put the steak in the freezer for 30 mins – it will firm up and be much easier to slice thinly. Meanwhile, make a marinade. Put the soy, sugar, mirin, ginger, garlic, pineapple, chilli flakes and oil in a food processor. Blitz until smooth, then pour into a bowl.
When the meat is semi-frozen, thinly slice into 25 pieces and add to the marinade. Chill until ready to use, but don’t leave for longer than 2 hrs or the pineapple juice will break down the meat too much.
Meanwhile, soak small wooden skewers in water for 10 mins. When ready to serve, thread a piece of meat onto each skewer. Heat a griddle pan, or frying pan, and brown skewers for 1-2 mins each side or until grill marks appear. Add a little more oil to the pan if needed.
Mix together the dipping sauce ingredients in a small bowl. Serve the skewers with lettuce, sesame seeds, spring onions and dipping sauce.