Prawn and avocado is the prawn sandwich given a rich soft partner, and what defines it is the meeting of two cool, yielding things that balance each other rather than compete. Small cold-water prawns, sweet and lean and faintly briny, are laid against ripe avocado, soft and fatty and almost flavourless on its own, on soft bread. The defining fact is the pairing logic: the prawn brings sweetness and a clean marine note but almost no fat; the avocado brings richness and body but little flavour of its own. Put together they complete each other, the avocado supplying the unctuousness a lean prawn lacks and the prawn supplying the savour a bland avocado lacks, which is why the combination reads as a single idea and not two fillings sharing bread.
The craft is ripeness and moisture control. The avocado has to be ripe enough to be soft and spread under light pressure but not so far gone that it goes grey and slack, because an underripe one is hard and tasteless and an overripe one collapses to a watery paste that floods the bread. It is sliced or roughly mashed and dressed with lemon, which both seasons it and slows the browning, and the lemon doubles as the acid that cuts the combined richness so the sandwich does not read as one heavy cool note. The prawns are kept whole so their snap survives against the soft avocado, the one textural break in a filling that is otherwise all give. A thin mayonnaise or Marie Rose is sometimes added to bind, but it is optional here because the avocado already supplies the fat that a bind would; where it is left out, the avocado is the bind. The bread is soft and plain, buttered lightly or not at all since the avocado carries the lubrication, and it is built close to eating because cut avocado oxidises and loses its point fast.
The variations move the partner or the bind. Prawn mayo and the plain prawn sandwich drop the avocado entirely; the prawn cocktail sandwich swaps the rich fat for a cold crisp leaf; prawn and Marie Rose leads on the sauce instead of the soft partner; the prawn salad sandwich widens the avocado into a fuller bed of salad vegetables. Each deserves its own article rather than being crowded in here.