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Put the cabbages in two separate small bowls, sprinkle each one with ½ tbsp of the vinegar and leave to marinate. Put the flour, egg and breadcrumbs onto separate plates. Mix all the ingredients for the tonkatsu sauce in a bowl and set aside.
Dust the pork or chicken in flour, dip in the egg on both sides and then into the breadcrumbs, making sure they are well covered. Heat 1cm of the oil in a deep frying pan until a breadcrumb sizzles as soon as it is dropped in. Fry the pork or chicken for 2-3 mins on each side or until the crumb coating is golden brown. Lift out and sprinkle with a little salt. Keep warm.
Dust, dip and coat the prawns in the same way. Add a little more oil to the pan if you need to and fry the prawns.
Spread mayonnaise over the slices of bread and lay them out on a board, then drizzle some tonkatsu sauce on two of the slices. Put the pork or chicken on one slice with tonkatsu on and arrange the prawns in a single layer on another. Spoon some pickled green cabbage onto the prawns and the pickled red cabbage onto the pork or chicken. Press another slice of bread, mayo-side down on each to make a sandwich. Trim the edges with a sharp knife and cut each one into four.