Sardines on toast gives up the second slice on purpose, and the thing that defines it is the oil going down into the toast rather than being kept out of bread. This is the open-face sardine dish: a single slice toasted firm, buttered while it is still hot, and the sardines from the tin laid on top whole or barely broken, often with the oil from the tin spooned over after them and the lot put briefly back under the grill until it sizzles. Where the closed sardine sandwich treats the oil as something to contain behind a butter seal, this one treats it as the entire point. The hot toast is built to be soaked. The oil, warmed and loosened, runs into the firm crumb and through it, carrying the salt and the savour of the fish down into the bread so the toast itself becomes part of the dish rather than a wrapper holding the fish off it.
The build works because the toast is taken further than it would be for a sandwich and the strong layer is kept thin. The bread is toasted firm and dry, well past pale, because a soft slice would collapse the instant the oil hit it whereas a properly crisp one drinks the oil and still holds its shape under a knife and fork. Butter goes on while the toast is hot so it melts in and meets the fish oil rather than sitting as a cold barrier, the opposite job from the butter in a closed sandwich. The sardines are left whole or only lightly crushed because here the texture of the fish is wanted against the crisp bread rather than mashed into a uniform paste, and they are laid on in a single restrained layer: the format has no second slice to balance a strong filling, so the whole effect rests on intense fish and warm oil against plain firm toast, and a heavy pile would just slide and overwhelm it. Lemon or vinegar and pepper finish it, the acid cutting the oil exactly as it does in the closed version.
The variations stay inside the single-slice, soaked-toast frame. The fish can be flashed under the grill for a blistered top or left as it comes from the tin; a dusting of cayenne or a smear of mustard sharpens it; mackerel or pilchards on hot toast are the same idea in a different oily fish. The closed, mashed sardine sandwich is the near relative that leads on thrift instead. Each of those deserves its own article rather than being crowded in here.