The prawn cocktail sandwich is the retro starter rebuilt between two slices, and what defines it is the full assembly rather than any one part: small cold prawns, Marie Rose sauce, and shredded lettuce, carried on soft brown bread. This is not prawns in plain mayonnaise and not the sauce on its own; it is the recognisable three-part set of the prawn cocktail, the pink-sauced prawns and the cold crisp leaf, transposed from a sundae glass into a sandwich and reading unmistakably as that dish the moment it is bitten. The lettuce is not a garnish here. It is one of the three structural elements, and removing it collapses the sandwich back into prawn mayo wearing a different sauce.
The craft is balance and moisture control across three wet-leaning components. The Marie Rose, mayonnaise tinted with tomato and sharpened with Worcestershire and lemon, is mixed thick enough to coat the prawns and cling rather than run, because a thin sauce floods the bread and turns the whole thing to a pink smear within minutes. The prawns are kept whole so their snap survives the sauce. The lettuce is shredded fine and, crucially, kept dry and patted of its own water, then layered as a deliberate cold, crisp counter to the soft sauced prawns, the same contrast the original starter runs on. Brown bread is the conventional carrier for the faint nuttiness that flatters sweet shellfish, buttered to the edges so the crumb is sealed against three things that all want to weep into it. It is assembled close to eating because the lettuce wilts and the sauce soaks the longer the three sit pressed together, which is why a good one is made to order and a chiller one rarely holds.
The variations split the assembly along its seams. Prawn and Marie Rose drops the lettuce and makes the sauce the entire point; prawn mayo strips it further to prawns in plain mayonnaise; the plain prawn sandwich removes the sauce altogether and lets the prawns sit loose under butter; prawn and avocado swaps the cold leaf for a rich soft partner; the prawn salad sandwich widens the lettuce into a fuller bed of salad vegetables. Each deserves its own article rather than being crowded in here.