Piroshki, written ピロシキ in Japanese, is a Russian-influenced filled bun that has settled comfortably into the Japanese bakery lineup. It sits in the broad sozai pan category of savory filled breads, a soft enriched dough wrapped around a seasoned meat filling and, in the Japanese style, more often deep-fried than baked. Whether to count it as a sandwich at all is a fair question, but it earns its place the way korokke pan and curry bread do: it is bread engineered around a filling, stocked on the same counters, and eaten the same way, in the hand on the move. The Japanese take keeps the Slavic idea while routing it through the local bakery sensibility for soft dough and tidy execution.
The craft is in the dough, the seal, and the fry. The dough is enriched and yeasted so it stays soft and slightly sweet, a deliberate counterpoint to the savory inside. The filling is typically ground meat cooked with onion and seasoning, sometimes with chopped egg, glass noodle, or cabbage folded through to lighten it; it has to be moist but not wet, since a runny filling steams the dough from within and turns it gummy. The seam is pinched tight and laid down so the bun does not split and weep oil in the fryer. Fried right, the crust comes out an even gold, crisp at first bite and yielding underneath, the inside hot and seasoned. A good piroshki is light for its size, savory, and clean on the fingers. A bad one is greasy and heavy from oil that was too cool, or doughy and bland inside when the filling was skimpy or underseasoned.
Bakeries diverge here in instructive ways. The fried version is the common one, but baked piroshki appear too, lighter and breadier with a softer crust. Fillings range from the meat-and-onion standard to versions with rice and egg, mushroom, or a curry-leaning mince that nods to Japan's own filled-bread habits. Some shops shape them long and others round, which changes the dough-to-filling ratio noticeably. The wider world of Japanese sozai pan, the savory filled-bread tradition this belongs to, is a deep subject that deserves its own article rather than being crowded in here.