Lifted from a tin, sardines arrive already cured, salted, and soft, the small fillets carrying the oil they were packed in, and the build is barely more than getting out of their way. The canonical assembly does almost nothing to them: a split loaf, a thick layer of butter, the sardines laid whole down the length, a few drops of lemon, a turn of black pepper. The work was done by whoever caught and tinned the fish; the sandwich is the assembly.
The logic follows from the fat and the salt. Sardines are an oily fish, so they bring their own richness and a strong saline edge, which is why butter sits underneath rather than mayonnaise or cheese: the butter bridges the salt to the wheat and rounds the fish without competing with it, while the lemon supplies the single acidic note that keeps the oil from going flat. The constraint is restraint. Pile on a sharp condiment or a loud cheese and you are fighting the cure instead of presenting it. The bread needs a real crust, because the soft fillets and their oil give the sandwich no structure and the loaf has to hold the shape while the oil soaks inward. It eats cool, clean, and best soon after it is built, before the oil saturates the crumb past the point of bite.
Variations stay within the tinned and grilled fish shelf. A version with fresh sardines, grilled and boned, is meatier and less saline than the tinned one; a build with a few slices of tomato adds a cool, sweet counterpoint to the salt; the plainest is sardines, butter, and bread alone. Each holds the oily fish as the fixed point and changes only how it arrives or what cuts it. The Sandwich aux Sardines belongs with the fish sandwiches the catalog groups under Baguette Poisson, the tradition that runs from oily tinned fish to smoked river and sea fish on bread and butter. Its specific contribution is a sandwich whose technique is almost no technique, where the tin did the curing and the cook only assembles.