Spread a thin layer of unsalted butter onto one side of all 4 slices of white milk bread. This adds flavor and creates a slightly toasted texture when the sandwich is assembled.
In a bowl, fold together cabbage and scallions. Sprinkle with sea salt, and add mayonnaise. Fold to combine, to finish the cabbage slaw.
Spoon a generous amount of cabbage slaw onto the buttered side of two slices of bread.
Place pieces of Pork Katsu on top of cabbage slaw.
Drizzle Tonkatsu (Katsu) sauce over each of the remaining two buttered slices of bread. Adjust the amount of sauce according to your taste preference.
Place the bread slices on top of the Pork Katsu, sauce-side down, to create the sandwiches.
Gently press the sandwiches together. You can trim the crusts if desired, for a more traditional presentation. Enjoy immediately or wrap it in parchment paper and refrigerate for later.
