Despite the similar name, the mikkusu sando is not the fruit one. This is the savory assortment: a single package containing two or three different small sandwiches, usually egg salad and ham, plus one rotating third, cut into triangles or rectangles and sold as a set. It is the workhorse of the convenience-store chiller and the neighborhood bakery's morning shelf, the thing people grab without deliberating because it covers several cravings at once and asks nothing of you. There is no single recipe, only a format and a set of expectations.
The appeal is in the contrast between the components, so each one has to hold its own. The egg salad is the anchor: hard-cooked eggs bound with Japanese mayonnaise into something creamy and a little sweet, sometimes with a touch of mustard, smooth rather than chunky in the konbini style. The ham portion is plain in the good sense, thin ham with a swipe of butter or mayo, occasionally a slice of cheese, on soft shokupan with the crusts cut away. The third member rotates: tuna and cucumber, ham and cheese, lettuce and tomato, potato salad, a thin cutlet. The bread has to stay soft without going wet, which is why the wetter fillings are kept thin and why the package is engineered to keep the cut faces from drying out. A good set has each sandwich distinct and well filled to the edges, the bread tender, the egg not weeping; a sloppy one has thin fillings that fade to the crust line, soggy bread under the tuna, or two near-identical sandwiches pretending to be three.
Variation is the whole genre. Every chain and bakery runs its own combination, and the third slot is where regional and seasonal ideas show up: shrimp salad, katsu, fruit-and-cream sneaking in as a sweet ringer, kimchi in some shops. Premium lines thicken the egg layer dramatically, swap in better ham, or use milk bread with a finer crumb. The format also scales up into party platters of mixed triangles for offices and gatherings. The egg salad component in particular, the tamago sando, has become its own object of devotion with its own arguments about jammy yolks and bread thickness, and it deserves its own article rather than being crowded in here.