The crayfish sandwich is the freshwater-tail build of the cocktail-sauce shellfish family, and what defines it is the swap at its centre: crayfish tails standing in where prawns would normally go. Cooked crayfish tails are folded into a cocktail sauce, the same pink, faintly sweet, lightly spiced Marie Rose dressing that carries a prawn cocktail, and laid on bread. The tail is the whole point of the substitution. It is firmer and meatier than a prawn, with a cleaner, slightly less briny taste, so the sandwich reads as a prawn cocktail rebuilt around a tail that holds its shape and its bite rather than going soft in the dressing.
The craft is the sauce and the moisture, the same problem the prawn cocktail sets. The cocktail sauce has to be sharp enough to cut the sweetness of the shellfish without slackening into a slick that soaks the bread, so it is bound thick and used in a measured amount rather than poured. The crayfish tails are drained well and folded in whole or roughly broken, never mashed, because the firmness of the tail is the texture the sandwich is built on and a crushed tail is just pink paste. A little crisp lettuce is often set under or through the filling as a structural counter, giving a soft, dressed filling something to push against. The bread is soft and plain, buttered to the edges so the crumb is sealed against a dressing that is, by design, slightly wet, and so nothing competes with a flavour built on sweet shellfish and sharp sauce. Cut and not overfilled, it stands on the tails being firm and the sauce being measured.
The variations are the rest of the cocktail-sauce shelf, each the same dressing met with a different catch. The prawn cocktail sandwich is the parent build with prawns in place of the tails; prawn mayonnaise drops the cocktail spice for a plain bound dressing; the crab and lobster builds carry richer shellfish through their own restraint. Each deserves its own article rather than being crowded in here.