The Sasami Katsu Sando is the lightest member of the katsu sando family, built not on a thick pork loin but on sasami, the chicken tenderloin: the long, narrow strip that runs along the inside of the breast, prized for being the leanest and most delicate cut on the bird. Crumbed, fried, and laid between soft white bread, it produces a sandwich with a much quieter center of gravity than its pork cousin. The whole point of this entry is restraint, and a kitchen either respects that or fights it.
The craft problem here is moisture. Sasami has almost no fat, which means it goes from juicy to dry in a very short window, so the cutlet has to be pounded gently to an even thinness, breaded in fine panko, and fried fast and hot enough that the crust sets before the meat overcooks. The bread is usually a tender shokupan with the crusts cut away, lightly buttered so a thin fat layer sits against the crumb. Sauce is the calibration: a restrained tonkatsu sauce or a light Japanese mayonnaise, used sparingly, because a heavy sweet glaze would bury a meat this subtle, while too little leaves the lean cutlet tasting plain. A good one is tender, faintly sweet from the crust, clean rather than rich, and cut into neat halves that show a pale, even cross-section. A sloppy one is a dry, stringy plank in a soggy crumb, the sasami overcooked into chalk and the sauce doing all the work.
Variations mostly turn on what is added to compensate for leanness without crowding it: a leaf of cabbage shredded fine for crunch, a smear of grain mustard for lift, a thin sheet of cheese for a richer build. Some shops fry the sasami in a lighter, almost tempura-style coat; others keep a thicker panko shell for contrast against the soft meat. The shiso-and-ume version, which threads perilla leaf and pickled-plum paste through the same cutlet to add tang and aroma, is a meaningfully different sandwich and deserves its own article rather than being crowded in here.