The Atherina (Αθερίνα) Sandwich is built around small fried fish, smelt, eaten whole and packed into bread, a coastal Greek way of turning a cheap netful into a handheld meal. The angle is the fish: atherina are tiny, eaten head, bones, and all, so the sandwich is really a delivery system for a crisp, briny, salty fry rather than a fillet construction. Done well it is light and shatteringly crisp; done poorly it is greasy and soft, the worst outcome for fried fish.
The build starts at the fryer, because nothing later can rescue a bad fry. The smelt are rinsed, drained well, then dredged, usually in seasoned flour, so the coating is thin and even with no wet clumps. They go into hot oil in small batches so the temperature holds and the fish crisp fast rather than stew; crowding the pan is the single most common ruin. Out of the oil they are drained hard and salted while still hot so the salt sticks. Only then does the bread come in: a roll or split loaf with enough structure to hold a pile of fish, dressed simply, often with lemon and maybe a sharp dairy or herb element to cut the oil. The fish go in hot and the sandwich is eaten immediately. Good execution shows a dry, audibly crisp exterior, fish that are not oily inside, clear salt, and bread that stays intact under the load. Sloppy versions read at once: pale underfried smelt that go limp, a thick gummy batter, oil soaking into the crumb, or fish salted too late so it tastes flat. Lemon at the end is not optional, since the acid is what keeps the whole thing from going heavy.
It shifts by coating and by what is added around the fry. A bare flour dredge eats lighter and more delicate; a heavier batter gives more crunch but risks greasiness if the oil is not hot enough. The dressing can stay minimal with just lemon, or lean on a yogurt-and-herb element for a creamier counterpoint. Battered salt cod packed into bread is a related but distinct fried-fish sandwich, and seafood folded into a flatbread is another preparation again; each deserves its own article rather than being crowded in here.