To make the marinade, heat the soy sauce, sesame oil, chilli, mirin, lime juice and rice wine in a small pot and bring to a gentle simmer. Take off the heat and stir in the white part of the spring onions and the garlic. Set aside to cool.
Season the beef with salt and pepper and let sit at room temperature for 10 minutes. Rub the beef with sesame oil, heat a frying pan on a high heat and sear the beef until well coloured all over but it should be very rare still. Allow the beef to cool a little and then pop into a resealable plastic bag with the marinade. Refrigerate overnight.
Remove beef from marinade, roll in the sesame seeds, pressing to coat. Using a sharp knife, slice the beef as thinly as possible.
Warm the tostadas according to packet instructions.
Top each tostada with lettuce, tomato and then a drizzle of Japanese mayo, top with sliced beef and a sprinkle of the reserved green spring onion.