The beef menchi katsu sando is the baseline ground-meat cutlet sandwich built on beef mince specifically, and the choice of meat is the whole point of distinguishing it. Menchi katsu is seasoned minced meat bound with onion, shaped into a patty, breaded in panko and deep-fried; swapping to beef gives the patty a deeper, more savory, slightly firmer character than the softer, sweeter pork build. Set between soft trimmed white bread with tonkatsu sauce, the construction is the standard one. What changes is the flavor in the center: beefier, richer, more like a fried hamburger steak than a croquette.
The craft is in keeping a beef mince juicy through the fryer. Beef sets firmer and can dry out faster than pork, so the mince is worked with onion and a little binder and fat to keep it loose and moist, formed into a patty, coated in fine panko, and fried until the shell is deep gold and crisp while the inside stays hot and running. It is brushed with tonkatsu sauce, dark, fruity, sweet and tangy, which plays well against beef's savor without burying it. The bread is thin soft shokupan, crusts off, sometimes buttered on the inner face so it absorbs the sauce instead of going limp, cut into clean pieces. The skill is in the doneness window: beef pushed too far in the fryer turns the patty tight and grey and the sandwich dry, while a well-judged one keeps a juicy, savory center that bleeds a little fat into the bread on purpose. Done well the eat is soft bread, a crisp shell, and a deep beefy core with the sauce sweet behind it. Done poorly the patty is dense and overcooked, the breading sogs, and the richness turns to grease.
Eating one is heartier and more savory than the plain or pork builds, closer in spirit to a fried hanbagu in bread, the beef reading clearly through the sweet sauce. It is still homestyle comfort food rather than anything refined, just with the dial turned toward meat.
The variations sit around it as siblings rather than offshoots. The plain baseline uses a mixed or unspecified mince; the pork version eats softer and sweeter; the cheese version melts a core through the meat. There is also the soft-roll form, menchi katsu pan, which carries the same patty in a hinged bun for one-handed eating. The pork-mince build, with its distinctly softer, sweeter character, deserves its own article rather than being crowded in here.