Pulse bread in food processor until medium crumbs form. Whisk cornstarch, salt and pepper in a shallow pan; place breadcrumbs in a second shallow pan. Whisk eggs in a third shallow pan. Dredge pork in cornstarch mixture, coat with egg then breadcrumb mixture, pressing breadcrumb mixture into pork to adhere. Place pork on a wire rack on a sheet tray; refrigerate up to 2 days.
Savoy Cabbage Slaw
Whisk vinegar, honey, ginger, tamari, mustard powder, salt, pepper and shiso in a medium bowl. Toss cabbage and cucumber with vinegar mixture; refrigerate in an airtight container up to 2 days. Makes about 2 cups.
Japanese Tartar Sauce
Stir all ingredients in a medium bowl; refrigerate in an airtight container up to 6 days. Makes about 1 cup.
Sweet Tamari Glaze
Heat all ingredients to a boil in a saucepan over medium-high heat, whisking frequently; reduce heat to low and simmer until reduced by ½, about 10 minutes, stirring occasionally. Refrigerate in an airtight container up to 6 days. Makes about 2/3 cup.
Deep-fry 1 piece pork at 375°F until golden brown and internal temperature reaches 145°F, about 3 minutes. Spread 1 side of 1 bread slice with 2½ tablespoons Japanese Tartar Sauce; top with Pork Katsu, 1 tablespoon Sweet Tamari Glaze, ½ cup Savoy Cabbage Slaw and remaining bread slice.
To make a vegetarian version of the sando, substitute medium-firm tofu for pork.