Take the lean, delicate sasami katsu sando and thread two sharp Japanese flavors through it, and you get this: a chicken-tenderloin cutlet sandwich layered with shiso, the green perilla leaf with its bright herbal-minty edge, and umeboshi paste, the salted, sour pickled plum pounded smooth. The additions are small in volume and enormous in effect. Where the plain sasami sando is a study in quiet, this one is built around a tart, aromatic jolt that runs straight down the middle of every bite.
The craft is the same fried-tenderloin discipline plus a balancing act. The sasami still has to be pounded even and fried fast and hot so the lean meat stays juicy under a fine panko crust, and the bread is still a soft, crust-trimmed shokupan, lightly buttered. The new work is dosing the ume. Umeboshi paste is intensely salty and sour, so it goes on as a thin restrained swipe, not a layer; too much and it overwhelms the mild chicken and turns the whole sandwich into pucker. The shiso leaves are laid whole or roughly torn against the cutlet so their aroma reads as fragrance rather than just more green. The mayonnaise or tonkatsu sauce, if used at all, is pulled back, because the ume is already supplying the acid the lean meat needs. A good one tastes clean, herbal, and brightly tart against warm crisp chicken, the plum cutting the fry without erasing it. A clumsy one is either a salt bomb that buries the sasami or a timid version where the shiso and ume barely register and you have paid attention for nothing.
Variations adjust the ratio and the carrier. Some kitchens fold the ume into the mayonnaise for a gentler, more diffuse tang; others keep a defined plum stripe for a sharper hit. Shiso is sometimes joined by a thin cucumber layer for extra cool crunch, or the cutlet is pressed for a crisper shell against the soft herbs. The plain sasami katsu sando without these accents is its own restrained thing and deserves its own article rather than being crowded in here.