The sardine sandwich is a tin turned into a meal, and the thing that defines it is thrift. A tin of sardines in oil is one of the cheapest forms of protein in a British cupboard, and the sandwich exists to make it into lunch with nothing else required: the fish is tipped out, the larger bones slipped away or simply mashed through, and the whole lot crushed with a fork into a coarse, oily paste, then spread thick on buttered bread between two slices. The mash is the defining move. A whole fillet laid flat slides and breaks unevenly and reads as a slab of strong fish; mashed, the sardine becomes a cohesive, spreadable layer that binds itself with its own oil, distributes its salt and its oiliness evenly across every bite, and stops being a fish you are confronted with and starts being a filling.
The build works because the oil is treated as the sauce rather than a problem to drain away. The sardine is intensely savoury, salty, and rich, so it needs an acid and a soft carrier and very little else: a squeeze of lemon or a few drops of malt vinegar worked into the mash cuts the oil and lifts the fish, and a grind of pepper is usually the whole of the seasoning. The bread is soft and plain because the filling brings all the flavour and any assertive loaf would only argue with it, and it is buttered to the edges, which here is structural as well as a matter of taste: the butter is a fat barrier that keeps the sardine oil from soaking straight through the crumb before the sandwich is eaten, and its salt bridges a very strong filling to a bland bread. Pressed gently and cut, it is a sandwich that rewards a generous, even layer and a real acid note and punishes a thin scrape or none.
The variations stay close to the tin. A whole-fillet version skips the mash for texture at the cost of stability; tomato or red onion goes in for a sharp counter; mashed pilchard or tinned mackerel is the same thrift met in a different oily fish. The open-face relative, sardines laid on hot buttered toast rather than mashed between cold slices, leads on the oil soaking the bread and is a different sandwich. Each of those deserves its own article rather than being crowded in here.